Objection 1. It would seem that vice is not contrary to nature. Because vice is contrary to virtue, as stated above (Article 1). Now virtue is in us, not by nature but by infusion or habituation, as stated above (63, A1,2,3). Therefore vice is not contrary to nature.
Objection 2. Further, it is impossible to become habituated to that which is contrary to nature: thus "a stone never becomes habituated to upward movement" (Ethic. ii, 1). But some men become habituated to vice. Therefore vice is not contrary to nature.
Objection 3. Further, anything contrary to a nature, is not found in the greater number of individuals possessed of that nature. Now vice is found in the greater number of men; for it is written (Matthew 7:13): "Broad is the way that leadeth to destruction, and many there are who go in thereat." Therefore vice is not contrary to nature.
Objection 4. Further, sin is compared to vice, as act to habit, as stated above (Article 1). Now sin is defined as "a word, deed, or desire, contrary to the Law of God," as Augustine shows (Contra Faust. xxii, 27). But the Law of God is above nature. Therefore we should say that vice is contrary to the Law, rather than to nature.
On the contrary, Augustine says (De Lib. Arb. iii, 13): "Every vice, simply because it is a vice, is contrary to nature."
I answer that, As stated above (Article 1), vice is contrary to virtue. Now the virtue of a thing consists in its being well disposed in a manner befitting its nature, as stated above (Article 1). Hence the vice of any thing consists in its being disposed in a manner not befitting its nature, and for this reason is that thing "vituperated," which word is derived from "vice" according to Augustine (De Lib. Arb. iii, 14).
But it must be observed that the nature of a thing is chiefly the form from which that thing derives its species. Now man derives his species from his rational soul: and consequently whatever is contrary to the order of reason is, properly speaking, contrary to the nature of man, as man; while whatever is in accord with reason, is in accord with the nature of man, as man. Now "man's good is to be in accord with reason, and his evil is to be against reason," as Dionysius states (Div. Nom. iv). Therefore human virtue, which makes a man good, and his work good, is in accord with man's nature, for as much as it accords with his reason: while vice is contrary to man's nature, in so far as it is contrary to the order of reason.
Reply to Objection 1. Although the virtues are not caused by nature as regards their perfection of being, yet they incline us to that which accords with reason, i.e. with the order of reason. For Cicero says (De Inv. Rhet. ii) that "virtue is a habit in accord with reason, like a second nature": and it is in this sense that virtue is said to be in accord with nature, and on the other hand that vice is contrary to nature.
Reply to Objection 2. The Philosopher is speaking there of a thing being against nature, in so far as "being against nature" is contrary to "being from nature": and not in so far as "being against nature" is contrary to "being in accord with nature," in which latter sense virtues are said to be in accord with nature, in as much as they incline us to that which is suitable to nature.
Reply to Objection 3. There is a twofold nature in man, rational nature, and the sensitive nature. And since it is through the operation of his senses that man accomplishes acts of reason, hence there are more who follow the inclinations of the sensitive nature, than who follow the order of reason: because more reach the beginning of a business than achieve its completion. Now the presence of vices and sins in man is owing to the fact that he follows the inclination of his sensitive nature against the order of his reason.
Reply to Objection 4. Whatever is irregular in a work of art, is unnatural to the art which produced that work. Now the eternal law is compared to the order of human reason, as art to a work of art. Therefore it amounts to the same that vice and sin are against the order of human reason, and that they are contrary to the eternal law. Hence Augustine says (De Lib. Arb. iii, 6) that "every nature, as such, is from God; and is a vicious nature, in so far as it fails from the Divine art whereby it was made."
Source: Summa Theologica (New Advent)