In the Sacrament of Matrimony, God sanctifies and elevates the natural love of a man and a woman in marriage. Jesus tells His disciples that this institution was originally intended by God to be a grace and blessing to mankind, and that God meant it to be permanent.
“But from the beginning of creation, ‘God made them male and female. For this reason, a man shall leave his father and mother (and be joined to his wife), and the two shall become one flesh.’ So, they are no longer two but one flesh. Therefore, what God has joined together, no human being must separate.” In the house the disciples again questioned him about this. He said to them, “Whoever divorces his wife and marries another commits adultery against her; and if she divorces her husband and marries another, she commits adultery.”Mark 10:6-12; see also Matthew 19:5-6
The teaching that marriage requires a permanent commitment is repeated by St. Paul.
Thus, a married woman is bound by law to her living husband; but if her husband dies, she is released from the law in respect to her husband. Consequently, while her husband is alive she will be called an adulteress if she consorts with another man. But if her husband dies she is free from that law, and she is not an adulteress if she consorts with another man.Romans 7:2-3; see also 1 Corinthians 7:10-11
Marriage, as a sacrament of the Church, symbolizes and confers God’s grace through a life of mutual love and sacrifice. In marriage, the husband and wife communicate God’s love through all aspects of their love for each other, and this love naturally produces and cares for their children. God extends His love through married Christians to build up the Church and sanctify the world.
Ultimately, the reason why marriage is a sacrament is that it symbolizes Christ’s love for Church.
Wives should be subordinate to their husbands as to the Lord. For the husband is head of his wife just as Christ is head of the church, he himself the savior of the body. As the church is subordinate to Christ, so wives should be subordinate to their husbands in everything. Husbands, love your wives, even as Christ loved the church and handed himself over for her to sanctify her, cleansing her by the bath of water with the word, that he might present to himself the church in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. So (also) husbands should love their wives as their own bodies. He who loves his wife loves himself. For no one hates his own flesh but rather nourishes and cherishes it, even as Christ does the church, because we are members of his body. “For this reason, a man shall leave (his) father and (his) mother and be joined to his wife, and the two shall become one flesh.” This is a great mystery, but I speak in reference to Christ and the church. In any case, each one of you should love his wife as himself, and the wife should respect her husband.Ephesians 5:22-33; see also Colossians 3:18-19
Just as Christ, the Heavenly Bridegroom gave Himself totally for His Bride, the Church, so a husband and his wife give themselves totally to each other. Just as Christ’s commitment to the Church is permanent, so marriage endures until death. In heaven, the blessed will be enjoying the wedding-feast of the Lamb (Revelation 19:7, 9), and so there will no longer be the need to symbolize this in marriage (see Matthew 22:23-30 and Mark 12:18-25). Thus, a marriage lasts until death.
A natural and holy part of the Sacrament of Matrimony is the sexual relationship between husband and wife.
Since God created [them] 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.’”Catechism of the Catholic Church, 1604
Since sex has a naturally tendency to produce children and to bring the couple closer together in love, the Church insists that all uses of sex must preserve these aspects if they are to be morally good. Any kind of sex outside of marriage (pre-marital, adulterous, masturbation and homosexual) and which intentionally and artificially frustrates the reproductive purpose of sex within marriage (artificial birth control or deliberate sterilization) are morally wrong and serious sins.
“The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose.” For here sexual pleasure is sought outside of “the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved.”CCC, 2352
The Church expects married couples to be responsible in the number of children they have.
For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood.CCC, 2368
The Church therefore permits birth control, so long as the method used does not artificially frustrate the reproductive tendency through chemical contraceptives, barriers or sterilization. “[T]he methods of birth regulation based on self-observation and the use of infertile periods is in conformity with the objective criteria of morality” (CCC 2370). These acceptable methods of birth control are often labeled Natural Family Planning (NFP).