another reason (2 Polit. ii): for since it is natural
that a man should have a liking for a woman of his kindred, if to
this be added the love that has its origin in venereal intercourse,
his love would be too ardent and would become a very great incentive
to lust: and this is contrary to chastity. Hence it is evident that
incest is a determinate species of lust.

Reply Obj. 1: Unlawful intercourse between persons related to one
another would be most prejudicial to chastity, both on account of the
opportunities it affords, and because of the excessive ardor of love,
as stated in the Article. Wherefore the unlawful intercourse between
such persons is called "incest" antonomastically.

Reply Obj. 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 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,