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 (A. 1) that the species of lust correspond to the
various conditions of women.

Reply Obj. 2: Matrimony is specially ordained for the good of human
offspring, as stated above (A. 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 Obj. 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 [* Cf. Suppl., Q. 59, A. 3; QQ. 60, 62], when we treat
of matrimony.
_______________________

NINTH ARTICLE [II-II, Q. 154, Art. 9]

Whether Incest Is a Determinate Species of Lust?

Objection 1: It would seem that incest is not a determinate species
of lust. For incest [* _Incestus_ is equivalent to _in-castus_ =
"unchaste"] takes its name from being a privation of chastity. But
all kinds of lust are opposed to chastity. Therefore it seems that
incest is not a species of lust, but is lust itself in general.

Obj. 2: Further, it is stated in the Decretals (XXXVI, qu. 1 [*Cf.
Append. Grat. ad can. Lex illa]) that "incest is intercourse between
a man and a woman related by consanguinity or affinity." Now affinity
differs from consanguinity. Therefore it is not one but several
species of lust.

Obj. 3: Further, that which does not, of itself, imply a deformity,
does not constitute a determinate species of vice. But intercourse
between those who are related by consanguinity or affinity does not,
of itself, contain any deformity, else it would never have been
lawful. Therefore incest is not a determinate species of lust.

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