12 For, whereas that natural use, when it pass beyond the compact of marriage, that is, beyond the necessity of begetting, is pardonable in the case of a wife, damnable in the case of an harlot; that which is against nature is execrable when done in the case of an harlot, but more execrable in the case of a wife. Of so great power is the ordinance of the Creator, and the order of Creation, that, in matters allowed us to use, even when the due measure is exceeded, it is far more tolerable, than, in what are not allowed, either a single, or rare excess.
And, therefore, in a matter allowed, want of moderation, in a husband or wife, is to be borne with, in order that lust break not forth into a matter that is not allowed. Hence is it also that he sins far less, who is ever so unceasing in approaches to his wife, than he who approaches ever so seldom to commit fornication. But, when the man shall wish to use the member of the wife not allowed for this purpose, the wife is more shameful, if she suffer it to take place in her own case, than if in the case of another woman.
Therefore the ornament of marriage is chastity of begetting, and faith of yielding the due of the flesh: this is the work of marriage, this the Apostle defends from every charge, in saying, “Both if you shall have taken a wife, you have not sinned: and if a virgin shall have been married, she sins not:” and, “Let her do what she will: she sins not if she be married.” But an advance beyond moderation in demanding the due of either sex, for the reasons which I have stated above, is allowed to married persons as matter of pardon.
Source: On the Good of Marriage (New Advent)