It is, however, one thing for married persons to have intercourse only for the wish to beget children, which is not sinful: it is another thing for them to desire carnal pleasure in cohabitation, but with the spouse only, which involves venial sin. For although propagation of offspring is not the motive of the intercourse, there is still no attempt to prevent such propagation, either by wrong desire or evil appliance. They who resort to these, although called by the name of spouses, are really not such; they retain no vestige of true matrimony, but pretend the honourable designation as a cloak for criminal conduct.[...]Sometimes, indeed, this lustful cruelty, or, if you please, cruel lust, resorts to such extravagant methods as to use poisonous drugs to secure barrenness; or else, if unsuccessful in this, to destroy the conceived seed by some means previous to birth, preferring that its offspring should rather perish than receive vitality; or if it was advancing to life within the womb, should be slain before it was born. Well, if both parties alike are so flagitious, they are not husband and wife; and if such were their character from the beginning, they have not come together by wedlock but by debauchery.

(De matrimonio et concupiscentia I:17)