according to their right proportion and meaning, is a fruit not only of the virtue which the couple practice, but also of the gifts of the Holy Spirit with which they cooperate. Marriage Marriage is a sacrament, and a public commitment between a man and a woman. Marriage builds the family and the society. The Church considers the expression of love between husband and wife to be an elevated form of human activity, joining husband and wife in complete, mutual self-giving, and opening their relationship to new life. As Pope Paul VI wrote in Humanae vitae, "The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council recalled, 'noble and worthy.'" Much of the Church's detailed doctrines derive from the principle that "sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive [between spouses] purposes". At the same time, the Bishops at Vatican II decreed that the essential procreative end of marriage does not make "the other purposes of matrimony of less account." Because sex is considered chaste only within context of marriage, it has come to be called the "nuptial act" in Catholic discourse. Among Catholics, the nuptial act is considered to be the conjoining of a man and a woman through sexual intercourse, considered an act of love between two married persons, and is considered in this way, a gift from God. When discussing chastity, the Catechism lists several transgressions and sins against it. The Church holds that the legal separation of spouses while maintaining the marriage bond can be legitimate in certain cases provided for by canon law. Sins Adultery One of the ten commandments states: "Do not commit adultery". The Catechism of the Catholic Church states that two partners commit adultery when they have sexual relations, even transient ones, while at least one of them is married to another party. There, adultery is defined as an injustice because it is an injury of the covenant of the marriage bond, a transgression of the other spouse, an undermining of the institution of marriage and a compromising of the welfare of children who need their parents' stable union. Child sex abuse and incest Incest and child sex abuse are counted as sins in the church's catechism in paragraphs 2388–2389. Contraception The Church has been opposed to contraception