Objection 1. It would seem that adultery is not a determinate species of lust, distinct from the other species. For adultery takes its name from a man having intercourse "with a woman who is not his own [ad alteram]," according to a gloss [St. Augustine: Serm. li, 13 de Divers. lxiii] on Exodus 20:14. Now a woman who is not one's own may be of various conditions, namely either a virgin, or under her father's care, or a harlot, or of any other description. Therefore it seems that adultery is not a species of lust distinct from the others.
Objection 2. Further, Jerome says [Contra Jovin. i]: "It matters not for what reason a man behaves as one demented. Hence Sixtus the Pythagorean says in his Maxims: He that is insatiable of his wife is an adulterer," and in like manner one who is over enamored of any woman. Now every kind of lust includes a too ardent love. Therefore adultery is in every kind of lust: and consequently it should not be reckoned a species of lust.
Objection 3. Further, where there is the same kind of deformity, there would seem to be the same species of sin. Now, apparently, there is the same kind of deformity in seduction and adultery: since in either case a woman is violated who is under another person's authority. Therefore adultery is not a determinate species of lust, distinct from the others.
On the contrary, Pope Leo [St. Augustine, De Bono Conjug. iv; Cf. Append. Grat. ad can. Ille autem. xxxii, qu. 5 says that "adultery is sexual intercourse with another man or woman in contravention of the marriage compact, whether through the impulse of one's own lust, or with the consent of the other party." Now this implies a special deformity of lust. Therefore adultery is a determinate species of lust.
I answer that, Adultery, as its name implies, "is access to another's marriage-bed [ad alienum torum]" [Cf. Append. Gratian, ad can. Ille autem. xxxii, qu. 1. By so doing a man is guilty of a twofold offense against chastity and the good of human procreation. First, by accession to a woman who is not joined to him in marriage, which is contrary to the good of the upbringing of his own children. Secondly, by accession to a woman who is united to another in marriage, and thus he hinders the good of another's children. The same applies to the married woman who is corrupted by adultery. Wherefore it is written (Sirach 23:32-33): "Every woman... that leaveth her husband... shall be guilty of sin. For first she hath been unfaithful to the law of the Most High" (since there it is commanded: "Thou shalt not commit adultery"); "and secondly, she hath offended against her husband," by making it uncertain that the children are his: "thirdly, she hath fornicated in adultery, and hath gotten children of another man," which is contrary to the good of her offspring. The first of these, however, is common to all mortal sins, while the two others belong especially to the deformity of adultery. Hence it is manifest that adultery is a determinate species of lust, through having a special deformity in venereal acts.
Reply to Objection 1. If a married man has intercourse with another woman, his sin may be denominated either with regard to him, and thus it is always adultery, since his action is contrary to the fidelity of marriage, or with regard to the woman with whom he has intercourse; and thus sometimes it is adultery, as when a married man has intercourse with another's wife; and sometimes it has the character of seduction, or of some other sin, according to various conditions affecting the woman with whom he has intercourse: and it has been stated above (Article 1) that the species of lust correspond to the various conditions of women.
Reply to Objection 2. Matrimony is specially ordained for the good of human offspring, as stated above (Article 2). But adultery is specially opposed to matrimony, in the point of breaking the marriage faith which is due between husband and wife. And since the man who is too ardent a lover of his wife acts counter to the good of marriage if he use her indecently, although he be not unfaithful, he may in a sense be called an adulterer; and even more so than he that is too ardent a lover of another woman.
Reply to Objection 3. The wife is under her husband's authority, as united to him in marriage: whereas the maid is under her father's authority, as one who is to be married by that authority. Hence the sin of adultery is contrary to the good of marriage in one way, and the sin of seduction in another; wherefore they are reckoned to differ specifically. Of other matters concerning adultery we shall speak in the Third Part [Supplement, 59, 3; Supplement, 60,62] when we treat of matrimony.
Source: Summa Theologica (New Advent)