Reply Obj. 3: There is something essentially unbecoming and contrary to natural reason in sexual intercourse between persons related by blood, for instance between parents and children who are directly and immediately related to one another, since children naturally owe their parents honor. Hence the Philosopher instances a horse (De Animal. ix, 47) which covered its own mother by mistake and threw itself over a precipice as though horrified at what it had done, because some animals even have a natural respect for those that have begotten them. There is not the same essential unbecomingness attaching to other persons who are related to one another not directly but through their parents: and, as to this, becomingness or unbecomingness varies according to custom, and human or Divine law: because, as stated above (A. 2), sexual intercourse, being directed to the common good, is subject to law. Wherefore, as Augustine says (De Civ. Dei xv, 16), whereas the union of brothers and sisters goes back to olden times, it became all the more worthy of condemnation when religion forbade it. _______________________
TENTH ARTICLE [II-II, Q. 154, Art. 1]
Whether Sacrilege Can Be a Species of Lust?
Objection 1: It would seem that sacrilege cannot be a species of lust. For the same species is not contained under different genera that are not subalternated to one another. Now sacrilege is a species of irreligion, as stated above (Q. 99, A. 2). Therefore sacrilege cannot be reckoned a species of lust.
Obj. 2: Further, the Decretals (XXXVI, qu. 1 [*Append. Grat. ad can. Lex illa]), do not place sacrilege among other sins which are reckoned species of lust. Therefore it would seem not to be a species of lust.
Obj. 3: Further, something derogatory to a sacred thing may be done by the other kinds of vice, as well as by lust. But sacrilege is not reckoned a species of gluttony, or of any other similar vice. Therefore neither should it be reckoned a species of lust.
On the contrary, Augustine says (De Civ. Dei xv, 16) that "if it is wicked, through covetousness, to go beyond one's earthly bounds, how much more wicked is it through venereal lust to transgress the bounds of morals!" Now to go beyond one's earthly bounds in sacred matters is a sin of sacrilege. Therefore it is likewise a sin of sacrilege to overthrow the bounds of morals through venereal desire in sacred matters. But venereal desire pertains to lust. Therefore sacrilege is a species of lust.
I answer that, As stated above (I-II, Q. 18, AA. 6, 7), the act of a virtue or vice, that is directed to the end of another virtue or vice, assumes the latter's species: thus, theft committed for the sake of adultery, passes into the species of adultery. Now it is evident that as Augustine states (De Virgin. 8), the observance of chastity, by being directed to the worship of God, becomes an act of religion, as in the case of those who vow and keep chastity. Wherefore it is manifest that lust also, by violating something pertaining to the worship of God, belongs to the species of sacrilege: and in this way sacrilege may be accounted a species of lust.
Reply Obj. 1: Lust, by being directed to another vice as its end, becomes a species of that vice: and so a species of lust may be also a species of irreligion, as of a higher genus.
Reply Obj. 2: The enumeration referred to, includes those sins which are species of lust by their very nature: whereas sacrilege is a species of lust in so far as it is directed to another vice as its end, and may coincide with the various species of lust. For unlawful intercourse between persons mutually united by spiritual relationship, is a sacrilege after the manner of incest. Intercourse with a virgin consecrated to God, inasmuch as she is the spouse of Christ, is sacrilege resembling adultery. If the maiden be under her father's authority, it will be spiritual seduction; and if force be employed it will be spiritual rape, which kind of rape even the civil law punishes more severely than others. Thus the Emperor Justinian says [*Cod. i, iii de Episc. et Cler. 5]: "If any man dare, I will not say to rape, but even to tempt a consecrated virgin with a view to marriage, he shall be liable to capital punishment."