7 But I marvel, if, as it is allowed to put away a wife who is an adulteress, so it be allowed, having put her away, to marry another. For holy Scripture causes a hard knot in this matter, in that the Apostle says, that, by commandment of the Lord, the wife ought not to depart from her husband, but, in case she shall have departed, to remain unmarried, or to be reconciled to her husband; whereas surely she ought not to depart and remain unmarried, save from an husband that is an adulterer, lest by withdrawing from him, who is not an adulterer, she cause him to commit adultery. But perhaps she may justly be reconciled to her husband, either he being to be borne with, if she cannot contain herself, or being now corrected. But I see not how the man can have permission to marry another, in case he have left an adulteress, when a woman has not to be married to another, in case she have left an adulterer. And, this being the case, so strong is that bond of fellowship in married persons, that, although it be tied for the sake f begetting children, not even for the sake of begetting children is it loosed. For it is in a man's power to put away a wife that is barren, and marry one of whom to have children. And yet it is not allowed; and now indeed in our times, and after the usage of Rome, neither to marry in addition, so as to have more than one wife living: and, surely, in case of an adulteress or adulterer being left, it would be possible that more men should be born, if either the woman were married to another, or the man should marry another. And yet, if this be not lawful, as the Divine Rule seems to prescribe, who is there but it must make him attentive to learn, what is the meaning of this so great strength of the marriage bond? Which I by no means think could have been of so great avail, were it not that there were taken a certain sacrament of some greater matter from out this weak mortal state of men, so that, men deserting it, and seeking to dissolve it, it should remain unshaken for their punishment. Seeing that the compact of marriage is not done away by divorce intervening; so that they continue wedded persons one to another, even after separation; and commit adultery with those, with whom they shall be joined, even after their own divorce, either the woman with a man, or the man with a woman. And yet, save in the City of our God, in His Holy Mount, the case is not such with the wife. But, that the laws of the Gentiles are otherwise, who is there that knows not; where, by the interposition of divorce, without any offense of which man takes cognizance, both the woman is married to whom she will, and the man marries whom he will. And something like this custom, on account of the hardness of the Israelites, Moses seems to have allowed, concerning a bill of divorcement. In which matter there appears rather a rebuke, than an approval, of divorce.
8. “Honorable,” therefore, “is marriage in all, and the bed undefiled.” And this we do not so call a good, as that it is a good in comparison of fornication: otherwise there will be two evils, of which the second is worse: or fornication will also be a good, because adultery is worse: for it is worse to violate the marriage of another, than to cleave unto an harlot: and adultery will be a good, because incest is worse; for it is worse to lie with a mother than with the wife of another: and, until we arrive at those things, which, as the Apostle says, “it is a shame even to speak of,” all will be good in comparison of what are worse. But who can doubt that this is false? Therefore marriage and fornication are not two evils, whereof the second is worse: but marriage and continence are two goods, whereof the second is better, even as this temporal health and sickness are not two evils, whereof the second is worse; but that health and immortality are two goods, whereof the second is better. Also knowledge and vanity are not two evils, whereof vanity is the worse: but knowledge and charity are two goods, whereof charity is the better. For “knowledge shall be destroyed,” says the Apostle: and yet it is necessary for this time: but “charity shall never fail.” Thus also this mortal begetting, on account of which marriage takes place, shall be destroyed: but freedom from all sexual intercourse is both angelic exercise here, and continues for ever. But as the repasts of the Just are better than the fasts of the sacrilegious, so the marriage of the faithful is to be set before the virginity of the impious. However neither in that case is repast preferred to fasting, but righteousness to sacrilege; nor in this, marriage to virginity, but faith to impiety. For for this end the righteous, when need is, take their repast, that, as good masters, they may give to their slaves, i.e., their bodies, what is just and fair: but for this end the sacrilegious fast, that they may serve devils. Thus for this end the faithful are married, that they may be chastely joined unto husbands, but for this end the impious are virgins, that they may commit fornication away from the true God. As, therefore, that was good, which Martha was doing, being engaged in the ministering unto the Saints, but that better, which Mary, her sister, sitting at the feet of the Lord, and hearing His word; thus we praise the good of Susanna in married chastity, but yet we set before her the good of the widow Anna, and, much more, of the Virgin Mary. It was good that they were doing, who of their substance were ministering necessaries unto Christ and His disciples: but better, who left all their substance, that they might be freer to follow the same Lord. But in both these cases of good, whether what these, or whether what Martha and Mary were doing, the better could not be done, unless the other had been passed over or left. Whence we are to understand, that we are not, on this account, to think marriage an evil, because, unless there be abstinence from it, widowed chastity, or virgin purity, cannot be had. For neither on this account was what Martha was doing evil, because, unless her sister abstained from it, she could not do what was better: nor on this account is it evil to receive a just man or a prophet into one's house, because he, who wills to follow Christ unto perfection, ought not even to have a house, in order to do what is better.
Source: On the Good of Marriage (New Advent)