Jesus said: "Have you not read the Scripture that says that in the beginning the Creator made people male and female? And God said, ‘For this reason a man will leave his father and mother and unite with his wife, and the two will become one.' So they are no longer two, but one" (Matt. 19:4-5). Although men and women had been getting married since the days of Adam and Eve, Christ made marriage a sacrament. The sacrament of matrimony is received when a baptized man and a baptized woman marry validly (CCC 1625).
A sacrament is an outward sign instituted by Christ to give inward grace. Not only are couples given an increase in sanctifying grace through the sacrament of matrimony, they are also given the assistance by God to help them be faithful to their vows and provide for and raise their children properly (CCC 1642), the family being the "domestic Church" (CCC 1655).
What are the duties of married couples? Husbands and wives have the duty to be faithful to their vows, and to have children: "Be fruitful and multiply" (Gen. 1:28). Husbands must "love (their) wives as Christ loves the Church" (Ehp. 5:25), while wives must "submit . . . to (their) husbands as to the Lord" (Eph. 5:22), because "a husband has authority over his wife, just as Christ has authority over the Church" (Ehp. 5:23). This does not mean that husbands are to "lord it over" their wives. Rather, there should be harmony between the respect a wife shows her husband and the love a husband shows his wife (Eph. 5:33).
Since God's first command to man was to "be fruitful and multiply", couples, in every sexual act, must be open to new life (CCC 2366). Therefore, it is morally unlawful for couples to practice artificial birth control (CCC 2368). The natural purpose of marital intercourse is both unitive (to bring the couple closer together) and procreative (to conceive children). To exclude one of the basic purposes of intercourse (procreation) by practicing birth control, is unnatural and unlawful. Sacred Scripture tells us that Onan was killed by God for engaging in birth control by withdrawal (Gen. 38:8-10). Sacred Tradition has always maintained that birth control is contrary to the natural law, a view held by all Christian churches until 1930. And the Magisterium has always taught, and continues to teach, that contraception is wrong. Because of this, many Catholic couples practice NFP (Natural Family Planning), which seeks to regulate the number of births by taking advantage of the fertile and infertile periods of the woman's cycle. Because it is natural to have intercourse during these times anyway, NFP is lawful, granted the couple has good reason to space births (CCC 2370). For the same reasons cited above, all forms of sterilization are unlawful (CCC 2379).
Some contend that homosexuality is permissible. The problem with this is that a marriage is only valid when a man marries a woman. Since two men or two women cannot marry each other, and since intercourse is only lawful within a marriage, homosexuality is not permitted. The other reason why homosexuality is not allowed is because such a union removes the procreative aspect of the sexual act from the realm of possibility (CCC 2357).
What about couples who experience difficulties in their marriage and seek a divorce? Jesus said: "No human must separate, then, what God has joined together" (Matt. 19:6). Jesus taught that no one could divorce: "Any man who divorces his wife for any cause other than her unfaithfulness, commits adultery if he marries another woman" (Matt. 19:9). Since marriage unites a man and a woman in the eyes of God for life, nothing but death can dissolve the marriage bond (CCC 1639). (Unfaithfulness in the above passage does not permit divorce and remarriage, it only permits couples to separate). There is only one exception, which is called the Pauline Privilege, named after Paul, who wrote that if two unbaptized persons marry, and one thereafter converts, while the other refuses to convert or allow the Christian to live peacefully in his newfound faith, the first marriage is dissolved upon the Christian breaking from his wife and marrying another (1 Cor. 12:17). Divorce is never possible (CCC 2384), although in some cases separation is permitted (CCC 2383). Even though the Church does not permit divorces, the Church does grant annulments. Nullity of marriage is declared when a supposed marriage is declared to be invalid (CCC 1629). Some examples of an invalid marriage include a man or a woman who did not fully and freely consent to marriage, who married when they were already validly married, who married without the intention of having children or without the intention of remaining faithful or staying married for life, a Catholic who married a non-Catholic outside the Catholic Church without a dispensation from the proper Catholic authorities (although two non-Catholics can validly married outside the Catholic Church; the Church discourages "mixed marriages", because it often results in the loss of Catholic faith by the Catholic partner, and because of the fear that children will not be raised Catholic [CCC 1634]), or without the presence of two witnesses, or a couple who never consummated their marriage. When an annulment is given, the man and the woman are free to marry, because through the annulment, the Church declares that they were never truly married in the first place.
Through Holy Orders, men receive a share in the priesthood of Christ, who is the "high priest" (Heb. 8:1-2), so that they may perform the sacred duties set apart for the ordained clergy–namely, to confer the sacraments and to bless. Priests are mediators, or go-betweens (CCC 1539). They intercede on the people's behalf, and through their intercession, God bestows his grace and blessings on the people. Priests, because they share in the priesthood of Christ, who is the "one mediator" (1 Tim. 2:5), are also mediators, although in a secondary way, and completely dependant upon the mediatorship of Christ. There are three levels of orders. Deacons have the power to bless, and can be the ordinary ministers of baptisms, weddings, and funerals (CCC 1570). Priests, while being able to do all that a deacon does, can also offer Mass by consecrating the Eucharist, absolve penitents from their sins, confirm, and anoint the sick. Bishops, who receive the fullness of the priesthood of Christ (CCC 1594), can ordain priests and other bishops. Priests in the Latin Rite of the Catholic Church take a vow of celibacy (CCC 1579), because "The unmarried man is busy with the Lord's affairs, concerned with pleasing the Lord; but the married man is busy with this world's demands and occupied with pleasing his wife. This means he is divided" (1 Cor. 7:32-33). Priests give up a wife and children so they can devote all their time and energy to serving the "Kingdom of God" (Matt. 19:12). Because the Church is the "bride of Christ", and Christ is the bridegroom, and because the priests stands "alter Christus"–in the place of Christ, it is his duty to serve the bride. Therefore, it is only appropriate that a man be a priest (CCC 1577), and therefore, Jesus ordained twelve apostles, while Paul would not "permit a woman to act as teacher [in other words, as a bishop who was a member of the Magisterium–the teaching office of the Church] or in any way to have authority over a man" (1 Tim. 2:12), two essential functions of the clergy. Priests promise obedience to their bishop, and religious priests take a vow of poverty.