Reply to Objection 2: Persons are related by affinity through one who is related by consanguinity: and therefore since the one depends on the other, consanguinity and affinity entail the same kind of unbecomingness.
Reply to Objection 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.
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.
Objection 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.
Objection 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 (FS, 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 to Objection 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 to Objection 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."
Reply to Objection 3: Sacrilege is committed on a consecrated thing. Now a consecrated thing is either a consecrated person, who is desired for sexual intercourse, and thus it is a kind of lust, or it is desired for possession, and thus it is a kind of injustice. Sacrilege may also come under the head of anger, for instance, if through anger an injury be done to a consecrated person. Again, one may commit a sacrilege by partaking gluttonously of sacred food. Nevertheless, sacrilege is ascribed more specially to lust which is opposed to chastity for the observance of which certain persons are specially consecrated.
Whether the unnatural vice is a species of lust?
Objection 1: It would seem that the unnatural vice is not a species of lust. For no mention of the vice against nature is made in the enumeration given above (A[1], OBJ[1]). Therefore it is not a species of lust.
Objection 2: Further, lust is contrary to virtue; and so it is comprised under vice. But the unnatural vice is comprised not under vice, but under bestiality, according to the Philosopher (Ethic. vii, 5). Therefore the unnatural vice is not a species of lust.
Objection 3: Further, lust regards acts directed to human generation, as stated above (Q[153], A[2]): Whereas the unnatural vice concerns acts from which generation cannot follow. Therefore the unnatural vice is not a species of lust.
On the contrary, It is reckoned together with the other species of lust ( 2 Cor. 12:21 ) where we read: "And have not done penance for the uncleanness, and fornication, and lasciviousness," where a gloss says: "Lasciviousness, i.e., unnatural lust."
I answer that, As stated above (AA[6],9) wherever there occurs a special kind of deformity whereby the venereal act is rendered unbecoming, there is a determinate species of lust. This may occur in two ways: First, through being contrary to right reason, and this is common to all lustful vices; secondly, because, in addition, it is contrary to the natural order of the venereal act as becoming to the human race: and this is called "the unnatural vice." This may happen in several ways. First, by procuring pollution, without any copulation, for the sake of venereal pleasure: this pertains to the sin of "uncleanness" which some call "effeminacy." Secondly, by copulation with a thing of undue species, and this is called "bestiality." Thirdly, by copulation with an undue sex, male with male, or female with female, as the Apostle states ( Rom. 1:27 ): and this is called the "vice of sodomy." Fourthly, by not observing the natural manner of copulation, either as to undue means, or as to other monstrous and bestial manners of copulation.
Reply to Objection 1: There we enumerated the species of lust that are not contrary to human nature: wherefore the unnatural vice was omitted.
Reply to Objection 2: Bestiality differs from vice, for the latter is opposed to human virtue by a certain excess in the same matter as the virtue, and therefore is reducible to the same genus.
Reply to Objection 3: The lustful man intends not human generation but venereal pleasures. It is possible to have this without those acts from which human generation follows: and it is that which is sought in the unnatural vice.
Whether the unnatural vice is the greatest sin among the species of lust?
Objection 1: It would seem that the unnatural vice is not the greatest sin among the species of lust. For the more a sin is contrary to charity the graver it is. Now adultery, seduction and rape which are injurious to our neighbor are seemingly more contrary to the love of our neighbor, than unnatural sins, by which no other person is injured. Therefore the unnatural sin is not the greatest among the species of lust.
Objection 2: Further, sins committed against God would seem to be the most grievous. Now sacrilege is committed directly against God, since it is injurious to the Divine worship. Therefore sacrilege is a graver sin than the unnatural vice.
Objection 3: Further, seemingly, a sin is all the more grievous according as we owe a greater love to the person against whom that sin is committed. Now the order of charity requires that a man love more those persons who are united to him---and such are those whom he defiles by incest---than persons who are not connected with him, and whom in certain cases he defiles by the unnatural vice. Therefore incest is a graver sin than the unnatural vice.
Objection 4: Further, if the unnatural vice is most grievous, the more it is against nature the graver it would seem to be. Now the sin of uncleanness or effeminacy would seem to be most contrary to nature, since it would seem especially in accord with nature that agent and patient should be distinct from one another. Hence it would follow that uncleanness is the gravest of unnatural vices. But this is not true. Therefore unnatural vices are not the most grievous among sins of lust.