Saint Jerome, Doctor and Father of the Church
Illicit sexual acts within marriage are equivalent to fornication and adultery, as Saint Jerome taught:
“And it makes no difference how honorable may be the cause of a man’s insanity. Hence Xystus in his Sentences tells us that ‘He who too ardently loves his own wife is an adulterer.’ It is disgraceful to love another man’s wife at all, or one’s own too much. A wise man ought to love his wife with judgment, not with passion. Let a man govern his voluptuous impulses, and not rush headlong into intercourse. There is nothing blacker than to love a wife as if she were an adulteress.” (St. Jerome, Against Jovinianus, Bk 1, n. 49)
Notice that St. Jerome states that “it makes no difference how honorable may be the cause of a man’s insanity.” In other words, the intention which motivates a man to sin is irrelevant to the morality of the act. If a sexual act is a sin, it does not matter how honorable the man’s intentions are, it is still a serious moral disorder, comparable, as a figure of speech, to the serious mental disorder of insanity.
St. Jerome plainly taught that there are sexual sins within marriage. The idea that “nothing is shameful” as long as the marital act occurs at some point in time is plainly rejected by St. Jerome. It is contrary to wisdom and good judgment for a man to have sexual relations with his wife in an inordinate manner. Though St. Jerome does not, like modern-day moral theologians, give explicit descriptions of various sexual acts, it is clear that he rejects the idea that the mere deposit of the right bodily fluid in the right location justifies all other acts.
In his commentary on the above verse, St. Jerome says: "In marriage, the acts that make for children are allowed; but the pleasures which are taken from the embraces of prostitutes are condemned in a wife."
In this quote, Jerome is not only condemning a man having sex with a prostitue, for he says that the acts in question are "condemned in a wife". What acts are these? They are sexual acts ordered toward pleasure without the procreative meaning. For these are sexual acts between husband and wife, which do not allow the possibility of procreation. In modern terms, these are sexual acts which are inherently non-procreative, making them intrinsically evil acts.
More on marital chastity in The Catholic Marriage Bed: A book of Roman Catholic moral theology.
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