 St. Thomas Aquinas (ca. 1225–1274) grounded concurring judgments about sexual morality in natural law, explicating marriage in terms of basic human goods, including procreation and fidelity between spouses (Finnis 1997). Monogamous marriage, as the arrangement fit for the rearing of children, “belong[s] to the natural law.” Monogamous marriage secures paternal guidance, which a child needs; fornication is thus a mortal sin because it “tends to injure the life of the offspring.” (Aquinas rejects polygamy on similar grounds while, like Augustine, arguing that it was once permitted to populate the earth.) Marital sex employs the body for its purpose of preserving the species, and pleasure may be a divinely ordained part of this. Even within marriage, sex is morally troubling because it involves “a loss of reason,” but this is compensated by the goods of marriage (Summa Theologiae, unfinished at Aquinas’ death, II-II, 153, 2; 154, 2). Among these goods, Aquinas emphasizes the mutual fidelity of the spouses, including payment of the “marriage debt” and “partnership of a common life”—a step towards ideas of companionate marriage (Summa Theologiae, Supp. 49, 1).