The Sixth Commandment “you shall not commit adultery” pertains not only to sexual infidelity but encompasses the whole of human sexuality (2336), and is thus perhaps the most controversial commandment among the Decalogue for a contemporary audience. In addressing the commandment, the Catechism begins on an anthropological note: the human person, created in the image of the Trinitarian God who is is love, is made to love (2231). Sexuality is an integral part of what it means to be made in the imago Dei. The human person is made from the beginning for love, for self-gift. This is what John Paul II called the “nuptial meaning of the body.” This capacity for self-gift is described in the Catechism and also reflected in John Paul II’s Theology of the Body as integrally connected to the importance of gender. It is in the union of man and woman that Trinitarian love is imaged: “Each of the two sexes is an image of the power and tenderness of God, with equal dignity though in a different way. The union of man and woman in marriage is a way of imitating in the flesh the Creator’s generosity and fecundity: ‘Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.'” (2335). Gender here is no accident. Rather, the distinction (and equality) of the sexes is part of the divine plan. In this way, the Catechism reflects a view known as “gender essentialism” which is closely connected with the idea of the complementarity of the sexes:

Everyone, man and woman, should acknowledge and accept his sexual identity. Physical, moral, and spiritual difference and complementarity are oriented toward the goods of marriage and the flourishing of family life. The harmony of the couple and of society depends in part on the way in which the complementarity, needs, and mutual support between the sexes are lived out (2333).

Sexuality is ordered toward the union of the man and woman in monogamous fidelity through the sacrament of marriage. In such a union, sex is not something dirty or shameful but is rather a source of good and holy joy. The purpose of sex within marriage is twofold: unity whereby the two are made one flesh, and procreation whereby the two imitate the life-giving love of the Creator (2363). These two ends of sex are integrally connected and cannot be severed. In addition, the marriage bond is marked by its indissolubility which God intends from the beginning: “what God has put together, let no man tear asunder.” As such, adultery and divorce are contrary to the natural law (2384). Civil divorce may be tolerated under certain circumstances dictated by canon law (2383), but remarriage is always illicit even when allowed on civil grounds. In like manner, polygamy, incest, and sexually active cohabitation outside of the bonds of marriage are also offenses against the dignity of marriage and the capacity of the human person for authentic self-gift.

Because the capacity to love is so deeply rooted in our affectivity, discipline is necessary. All are called to the virtue of chastity which is lived out specifically according to one’s state in life (2348). Chastity is not, as it is commonly assumed, the same as abstinence. Married, single, and celibate alike are called to chastity which involves training one’s emotions to love with right reason. Chastity is a part of the cardinal virtue of temperance which is the virtue governing our “concupiscible appetite,” that is, our appetite for created goods (food, drink, sex). As an acquired virtue, chastity takes a long time and much effort to develop (2342), and at times of intense sexual development (i.e. adolescence) or times of sexual temptation, it will need to be cultivated and protected by the virtue of fortitude.

Vices against chastity include anything that allows one’s reason to be conquered by one’s sexual feelings or fails to fully reflect the capacity for authentic self-gift as fitting the imago Deit. The disordinate desire for sexual pleasure (lust) is the root of a host of other vices: masturbation, fornication, pornography, rape. In each of these vices, sex is used for something other than self-giving love and the body is objectified—treated as a thing for use and not an integral part of a person.

A highly disputed vice against chastity is the sexual act among those of the same sex. The pastoral tone of the Catechism is evident here, acknowledging that the number of those experiencing sexual desire for a member of the same sex is not insignificant (2358) and that those experiencing such desire are to be treated with sensitivity and compassion. Nevertheless, the Catechism affirms that such relations are intrinsically disordered as contrary to the natural law, the complementarity of the sexes, and the necessary fecundity of the sexual act (2357). As with all the baptized, homosexual people are called to chastity and to strive for Christian perfection through mastery of their sexual desire (2359).

The biggest problem with same-sex relations is that they cannot, in the eyes of the Church, achieve the twofold purpose of sex: unity and procreation. The conjugal act is ordered toward fecundity (2366) and any sexual act which intentionally denies this end is illicit. The interdependence of unity and procreation provide the basis of the Church’s opposition to artificial contraception (2370). Acts which are intended for the regulation or spacing of births are justified only if they conform with objective criteria. Accordingly, the Church supports periodic continence (abstaining from sex) but forbids other forms of contraception including barrier (condoms, diaphragms) and hormonal (the Pill) methods as intentionally trying to preclude the procreative dimension of the sexual act.

Despite the importance of fecundity, it is important to note that a child is a gift, the fruit of Trinitarian creative love, not a right (2378). Again, the Catechism adopts a pastoral tone towards those couples experiencing infertility, and scientific and medical solutions are encouraged so long as they conform with the moral law and the rights of personhood for the child in question. Acts which sever the marital bond through the use of a third party (an egg or sperm donor or surrogate) are called heterologous and violate the bond of fidelity within the marriage ( 2376). Such acts are illicit not only as violations of fidelity, but also because they deny a child the right to two parents. On the other hand, homologous reproductive therapies (IVF and artificial insemination) which use only the reproductive materials of the spouses are more understandable but still a violation of the moral law because they sever the unitive and procreative dimension of the sexual act (2378). Permanently infertile couples are encouraged to adopt or to find other ways of living out authentic self-gifting love.

On a final note, although not fleshed out explicitly in this part of the Catechism, it is important to note that it is in celibacy, not in marriage, that the human person finds her highest state in life (1 Cor 7). While marriage is part of the divine plan and is ordained from the beginning, self-gifting love finds its fullest expression outside of the sexual act. The celibate are enabled to “give themselves to God alone with an undivided heart in a remarkable manner” (2349). The vow of celibacy taken by men and women religious ought not to be seen as a “no” to sexuality, but rather as a radical “yes” to the ultimate purpose of sexuality—to be united spiritually and corporeally to the God who made us like Himself and calls us to be eternally united to him in love.