Augustine of Hippo, On the Good of Marriage Chapter 6

6. Further, in the very case of the more immoderate requirement of the due of the flesh, which the Apostle enjoins not on them by way of command, but allows to them by way of leave, that they have intercourse also beside the cause of begetting children; although evil habits impel them to such intercourse, yet marriage guards them from adultery or fornication. For neither is that committed because of marriage, but is pardoned because of marriage. Therefore married persons owe one another not only the faith of their sexual intercourse itself, for the begetting of children, which is the first fellowship of the human kind in this mortal state; but also, in a way, a mutual service of sustaining one another’s weakness, in order to shun unlawful intercourse: so that, although perpetual continence be pleasing to one of them, he may not, save with consent of the other. For thus far also, “The wife hath not power of her own body, but the man: in like manner also the man hath not power of his own body, but the woman.” That that also, which, not for the begetting of children, but for weakness and incontinence, either he seeks of marriage, or she of her husband, they deny not the one or the other; lest by this they fall into damnable seductions, through temptation of Satan, by reason of incontinence either of both, or of whichever of them. For intercourse of marriage for the sake of begetting hath not fault; but for the satisfying of lust, but yet with husband or wife, by reason of the faith of the bed, it hath venial fault: but adultery or fornication hath deadly fault, and, through this, continence from all intercourse is indeed better even than the intercourse of marriage itself, which takes place for the sake of begetting. But because that Continence is of larger desert, but to pay the due of marriage is no crime, but to demand it beyond the necessity of begetting is a venial fault, but to commit fornication or adultery is a crime to be punished; charity of the married ought to beware, lest whilst it seek for itself occasion of larger honor, it do that for its partner which cause condemnation. “For whosoever putteth away his wife, except for the cause of fornication, maketh her to commit adultery.” To such a degree is that marriage compact entered upon a matter of a certain sacrament, that it is not made void even by separation itself, since, so long as her husband lives, even by whom she hath been left, she commits adultery, in case she be married to another: and he who hath left her, is the cause of this evil.