“The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life and which is ordered by its nature to the good of the spouses and the procreation and education of offspring, has been raised by Christ the Lord to the dignity of a sacrament between the baptized" (Code of Canon Law , 1055 §1).
1. The divine plan for marriage
“God himself is the author of marriage."  The intimate conjugal union between a man and a woman is sacred, and is structured according to laws established by the Creator and independent of human choice.
The institution of marriage is not an undue interference in the intimate personal relations between a man and a woman, but an internal requirement of the covenant of conjugal love. It is only in marriage that the love between a man and a woman can be conjugal,  that is, an elective love that embraces the good of the entire person in as much as sexually differentiated.  “Since God created [mankind] as man and woman, their mutual love becomes an image of the absolute and unfailing love with which God loves man. It is good, very good, in the Creator's eyes. And this love which God blesses is intended to be fruitful and to be realized in the common work of watching over creation: 'And God blessed them, and God said to them: Be fruitful and multiply, and fill the earth and subdue it' ( Gen 1:28)" (CCC, 1604).
The original communion between man and woman was ruptured by original sin, which weakened the moral conscience's ability to recognize the unity and indissolubility of marriage. Old Testament Law, in accord with Divine pedagogy, did not condemn the polygamy of the patriarchs nor did it forbid divorce. Rather, “seeing God's covenant with Israel in the image of exclusive and faithful married love, the prophets prepared the Chosen People's conscience for a deepened understanding of the unity and indissolubility of marriage. The books of Ruth and Tobit bear moving witness to an elevated sense of marriage and to the fidelity and tenderness of spouses. Tradition has always seen in the Song of Solomon a unique expression of human love, insofar as it is a reflection of God's love—a love 'strong as death' that 'many waters cannot quench'" (CCC , 1611).
“Christ not only restored the original order of matrimony but raised it to the dignity of a sacrament, giving spouses a special grace to live out their marriage as a symbol of Christ's love for his bride the Church: 'Husbands, love your wives, as Christ loved the Church' " (Eph 5:25) (Compendium , 341).
“For this reason, a valid matrimonial contract cannot exist between the baptized without it being by that fact a sacrament" (Code of Canon Law , 1055 §2). 
The sacrament of matrimony increases sanctifying grace and confers a specific sacramental grace, which exercises a strong influence on all the realities of married life,  especially on the love between the spouses.  The universal vocation to sanctity is specified for married people “by the sacrament they have celebrated and is carried out concretely in the realities proper to their conjugal and family life."  “Husband and wife are called to sanctify their married life and to sanctify themselves in it. It would be a serious mistake if they were to exclude family life from their spiritual development. The marriage union, the care and education of children, the effort to provide for the needs of the family as well as for its security and development, the relationships with other persons who make up the community, all these are among the ordinary human situations that Christian couples are called upon to sanctify." 
2. The celebration of matrimony
Matrimony arises from the personal and irrevocable consent of the spouses (cf. CCC , 1626). “Matrimonial consent is an act of the will by which a man and a woman mutually give and accept each other through an irrevocable covenant in order to establish marriage" (Code of Canon Law , 1057 §2).
“The Church normally requires that the faithful contract marriage according to the ecclesiastical form" (CCC, 1631). Hence “only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses according to the rules" set forth in the Code of Canon Law (Code of Canon Law , 1108 §1).
There are several reasons for this public celebration: the sacrament of matrimony is a liturgical act, and introduces spouses into an ecclesial order , creating rights and duties in the Church between the spouses and towards their children. Since matrimony is a state of life within the Church, certainty about it is necessary (hence the need for witnesses). And the public character of the consent protects the 'I do' once given and helps the spouses to remain faithful to it (cf. CCC , 1631).
3. Essential properties of marriage
“The essential properties of marriage are unity and indissolubility, which in Christian marriage obtain a special firmness by reason of the sacrament" ( Code of Canon Law , 1056). Husband and wife “'are no longer two, but one' ( Mt 19:8) … The intimate union of marriage, as a mutual giving of two persons, and the good of the children demand total fidelity from the spouses and require an unbreakable unity between them." 
“The unity of marriage, distinctly recognized by our Lord, is made clear in the equal personal dignity which must be accorded to man and wife in mutual and unreserved affection. Polygamy is contrary to conjugal love which is undivided and exclusive" (CCC , 1645).
“In his preaching Jesus unequivocally taught the original meaning of the union of man and woman as the Creator willed it from the beginning (cf. Mt 19:8): permission given by Moses to divorce one's wife was a concession to the hardness of hearts. The matrimonial union of man and woman is indissoluble. God himself has determined it: 'what therefore God has joined together, let no man put asunder' (Mt 19:6)" (CCC , 1614).In virtue of the sacrament, by which Christian spouses witness to and participate in the mystery of the union and fruitful love between Christ and the Church (cf. Eph 5:32), the indissolubility acquires a new and deeper meaning, strengthening the solidity of the marriage bond, so that a “marriage which is ratified [that is, celebrated between baptized persons] and consummated cannot be dissolved by any human power or by any cause other than death" (Code of Canon Law , 1141).
“Divorce is a grave offence against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other until death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign" (CCC, 2384). “It can happen that one of the spouses is an innocent victim of a divorce decreed by civil law; this spouse therefore has not contravened the moral law. There is considerable difference between a spouse who has sincerely tried to be faithful to the sacrament of marriage and is unjustly abandoned, and one who through his own grave fault destroys a canonically valid marriage" (CCC, 2386).
“Yet there are some situations in which living together becomes practically impossible for a variety of reasons. In such cases the Church permits the physical separation of the couple and their living apart. The spouses do not cease to be husband and wife before God and so are not free to contract a new union. In this difficult situation, the best solution would be, if possible, reconciliation" (CCC, 1649) If after the separation “civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense" ( CCC, 2383).
If after divorce a new union is contracted, even though it may be recognized in civil law, it “adds to the gravity of the rupture: the remarried spouse is then in a situation of public and permanent adultery" (CCC, 2384). Divorced persons who remarry, even though they continue belonging to the Church, may not be readmitted to the Eucharist, because their state and condition of life objectively contradicts the union of indissoluble love between Christ and the Church, which is signified and made present in the Eucharist. “Reconciliation in the sacrament of Penance which would open the way to the Eucharist, can only be granted to those who, repenting of having broken the sign of the Covenant and of fidelity to Christ, are sincerely ready to undertake a way of life that is no longer in contradiction to the indissolubility of marriage. This means, in practice, that when, for serious reasons, such as for example the children's upbringing, a man and a woman cannot satisfy the obligation to separate, they 'take on themselves the duty to live in complete continence, that is, by abstinence from the acts proper to married couples.'" 
4. Responsible parenthood
“By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory. Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: 'It is not good that man should be alone,' (Gen 2:18) and 'from the beginning [He] made them male and female' (Mt 19:4); wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: 'Be fruitful and multiply' (Gen 1:28). Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing the spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day" (CCC, 1652).  Therefore “among the married couples who thus fulfill their God-given mission, special mention should be made of those who, after prudent reflection and common decision, courageously undertake the proper upbringing of a large number of children." 
The stereotype of the family presented by present-day culture is opposed to large families, which is seen as justified by economic, social or other reasons. However, “true mutual love transcends the union of husband and wife and extends to its natural fruit—the children. Selfishness, on the contrary, sooner or later reduces love to a mere satisfaction of instinct and destroys the bond uniting parents and children. A child who suspects that he has come into the world against his parents' will, who feels he was born not of a pure love, but because of miscalculation or oversight, can hardly consider himself a good son—a true son—of his parents … I see clearly that attacks on large families stem from a lack of faith. They are the product of a social atmosphere which is incapable of understanding generosity, trying to conceal selfishness and unmentionable practices under apparently altruistic motives." 
Even though spouses may be ready to be generous about the number of children they have, “married people are often hindered by certain situations in modern life from working out their married life harmoniously and … can sometimes find themselves in a position where the number of children cannot be increased, at least for the time being."  “If therefore there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling births." 
Any action is intrinsically evil “which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation—whether as an end or as a means." 
Although it is meant to delay a new conception, the moral value of the conjugal act carried out during the woman's infertile period is different from the moral value of a marital act carried out using some form of contraception. “The fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which man is called." 
The use of contraceptives excludes the procreative meaning of the use of marriage, while the conjugal act during the woman's infertile periods respects the inseparable connection between the unitive and procreative elements of human sexuality. In the first case, a positive action is taken to prevent procreation, eliminating from the conjugal act its proper power of procreating. In the second, the use of marriage is avoided only during the woman's fertile periods, which doesn't damage the procreative power of any conjugal act. 
Hence, responsible parenthood, as taught by the Church, is in no sense contraceptive. On the contrary, it is the response to a particular situation arising from circumstances that aren't desired but are in some sense forced on the couple, and that can contribute (when there is prayer and the acceptance of God's will) to uniting more firmly both the spouses and the whole family.
5. Matrimony and the family
“According to the plan of God, marriage is the foundation of the wider community of the family, since the very institution of marriage and conjugal love are ordained to the procreation and education of children, in whom they find their crowning." 
“Since the Creator of all things has established the conjugal partnership as the beginning and basis of human society," the family is “the first and vital cell of society."  This specific public dimension of marriage and the family entails that civil authorities must defend and foster it.  Laws that fail to recognize the essential properties of marriage (laws that permit divorce, or that equate civil unions, whether heterosexual or between persons of the same sex, to marriage) are unjust; they gravely damage the fabric of society itself, which the state is obliged to protect and foster. 
In the Church the family is called “the domestic church;" the communion of its members is meant to be “a specific revelation and realization of ecclesial communion."  “Parents, by word and example, are the first heralds of the faith to their children. They must foster the vocation which is proper to each child, and this with special care if it be to religion."  “It is here that the father of the family, the mother, children, and all members of the family exercise the priesthood of the baptized in a privileged way by the reception of the sacraments, prayer and thanksgiving, the witness of a holy life, and self-denial and active charity. Thus the home is the first school of Christian life and a school for human enrichment. Here one learns endurance and the joy of work, fraternal love, generous—even repeated—forgiveness, and above all divine worship in prayer and the offering of one's life" (CCC, 1657).
Catechism of the Catholic Church , (CCC), 1601- 1666, 2331- 2400.
Second Vatican Council, Const. Gaudium et Spes , 47-52.
John Paul II, Apost. Exhort. Familiaris Consortio , November 22, 1981, 11-16.
St. Josemaria, Conversations , 87-112; also the homily, Marriage: a Christian Vocation , in Christ is Passing By.
 Second Vatican Council, Const. Gaudium et Spes , 48.
 Cf. John Paul II, Apost. Exort. Familiaris Consortio , 11.
 Second Vatican Council, Gaudium et Spes , 49.
 “Indeed, by means of baptism, man and woman are definitively placed within the new and eternal covenant, in the spousal covenant of Christ with the Church. And it is because of this indestructible insertion that the intimate community of conjugal life and love, founded by the Creator, is elevated and assumed into the spousal charity of Christ, sustained and enriched by His redeeming power" (John Paul II, Familiaris Consortio, 13).
 “Couples have the grace of the married state—the grace they receive in the sacrament of Marriage—which enables them to live all the human and Christian virtues in their married life: understanding, good humor, patience, forgiveness, refinement and consideration in their mutual relations" (St Josemaria, Conversations , 108).
 “Authentic married love is caught up into divine love and is directed and enriched by Christ's redeeming power and the salvific action of the Church, so that this love may lead the spouses to God with powerful effect and may aid and strengthen them in their lofty role as a father or a mother" (Second Vatican Council, Gaudium et Spes , 48).
 John Paul II, Familiaris Consortio , 56.
 St. Josemaria, Christ is Passing By , 23.
 Second Vatican Council, Gaudium et Spes , 48.
 John Paul II, Familiaris Consortio , 84. See also Benedict XVI, Apost. Exort. Sacramentum Caritatis , February 22, 2007, 29; Congregation for the Doctrine of the Faith, Letter on the reception of Holy Communion by divorced among the faithful who have remarried, September 14,1994; and CCC, 1650.
 “Married couples should regard it as their proper mission to transmit human life and to educate their children ... Whenever Christian spouses in a spirit of sacrifice and trust in divine providence carry out their duties of procreation with generous human and Christian responsibility, they glorify the Creator and perfect themselves in Christ" (Second Vatican Council, Gaudium et Spes , 50).
 Ibid .
 St. Josemaria, Conversations , 94. “A married couple should build their life together on the foundation of a sincere and pure affection for each other, and on the joy that comes from having brought into the world the children God has enabled them to have. They should be capable of renouncing their personal comfort; and they should put their trust in the providence of God. To have a large family—if such is the will of God—is a guarantee of happiness and effectiveness, in spite of everything that the mistaken proponents of a life based on selfish pleasures may say to the contrary" (St. Josemaria, Christ is Passing By , 25).
 Second Vatican Council, Gaudium et Spes , 51.
 Paul VI, Enc. Humanae Vitae , July 26, 1968, 16.
 Ibid ., 14.
 Ibid ., 12. The marriage act, when carried out excluding one of these essential qualities, is intrinsically disordered. “Men rightly observe that a conjugal act imposed on one's partner without regard to his or her condition or personal and reasonable wishes in the matter, is no true act of love, and therefore offends the moral order in its particular application to the intimate relationship of husband and wife. If they further reflect, they must also recognize that an act of mutual love which impairs the capacity to transmit life which God the Creator, through specific laws, has built into it, frustrates His design which constitutes the norm of marriage, and contradicts the will of the Author of life. Hence to use this divine gift while depriving it, even if only partially, of its meaning and purpose, is equally repugnant to the nature of man and of woman, and is consequently in opposition to the plan of God and His holy will" ( Ibid ., 13).
 Cf. John Paul II, Familiaris Consortio , 32; and CCC , 2370. The suppression of the procreative meaning brings with it the elimination of the unitive meaning of the marital act: “this leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality" ( Familiaris Consortio , 32).
 Ibid ., 14.
 Ibid ., 42.
 “The family is the natural and fundamental element of society and has the right to the protection of society and of the state" (United Nations, Universal Declaration of Human Rights , December 10, 1948, art. 16).
 Cf. Pontifical Council for the Family, Family, Marriage and Irregular Partnerships , Vatican City 2000. Congregation for the Teaching of the Faith; Considerations on Projects for the Legalization of Unions of Persons of the Same Sex., Vatican City 2003.  John Paul II, Familiaris Consortio , 21.
 Second Vatican Council, Lumen Gentium , 11.