Objection 1. It would seem that the precepts of temperance are unsuitably given in the Divine law. Because fortitude is a greater virtue than temperance, as stated above (123, 12; 141, 8; I-II, 66, 4). Now there is no precept of fortitude among the precepts of the decalogue, which are the most important among the precepts of the Law. Therefore it was unfitting to include among the precepts of the decalogue the prohibition of adultery, which is contrary to temperance, as stated above (154, 1,8).
Objection 2. Further, temperance is not only about venereal matters, but also about pleasures of meat and drink. Now the precepts of the decalogue include no prohibition of a vice pertaining to pleasures of meat and drink, or to any other species of lust. Neither, therefore, should they include a precept prohibiting adultery, which pertains to venereal pleasure.
Objection 3. Further, in the lawgiver's intention inducement to virtue precedes the prohibition of vice, since vices are forbidden in order that obstacles to virtue may be removed. Now the precepts of the decalogue are the most important in the Divine law. Therefore the precepts of the decalogue should have included an affirmative precept directly prescribing the virtue of temperance, rather than a negative precept forbidding adultery which is directly opposed thereto.
On the contrary, stands the authority of Scripture in the decalogue (Exodus 20:14-17).
I answer that, As the Apostle says (1 Timothy 1:5), "the end of the commandment is charity," which is enjoined upon us in the two precepts concerning the love of God and of our neighbor. Wherefore the decalogue contains those precepts which tend more directly to the love of God and of our neighbor. Now among the vices opposed to temperance, adultery would seem most of all opposed to the love of our neighbor, since thereby a man lays hold of another's property for his own use, by abusing his neighbor's wife. Wherefore the precepts of the decalogue include a special prohibition of adultery, not only as committed in deed, but also as desired in thought.
Reply to Objection 1. Among the species of vices opposed to fortitude there is not one that is so directly opposed to the love of our neighbor as adultery, which is a species of lust that is opposed to temperance. And yet the vice of daring, which is opposed to fortitude, is wont to be sometimes the cause of murder, which is forbidden by one of the precepts of the decalogue: for it is written (Sirach 8:18): "Go not on the way with a bold man lest he burden thee with his evils."
Reply to Objection 2. Gluttony is not directly opposed to the love of our neighbor, as adultery is. Nor indeed is any other species of lust, for a father is not so wronged by the seduction of the virgin over whom he has no connubial right, as is the husband by the adultery of his wife, for he, not the wife herself, has power over her body [1 Corinthians 7:4.
Reply to Objection 3. As stated above (122, 1,4) the precepts of the decalogue are universal principles of the Divine law; hence they need to be common precepts. Now it was not possible to give any common affirmative precepts of temperance, because the practice of temperance varies according to different times, as Augustine remarks (De Bono Conjug. xv, 7), and according to different human laws and customs.
Source: Summa Theologica (New Advent)