46 Moreover, if unbelief is fornication, and idolatry unbelief, and covetousness idolatry, it is not to be doubted that covetousness also is fornication. Who, then, in that case can rightly separate any unlawful lust whatever from the category of fornication, if covetousness is fornication? And from this we perceive, that because of unlawful lusts, not only those of which one is guilty in acts of uncleanness with another's husband or wife, but any unlawful lusts whatever, which cause the soul making a bad use of the body to wander from the law of God, and to be ruinously and basely corrupted, a man may, without crime, put away his wife, and a wife her husband, because the Lord makes the cause of fornication an exception; which fornication, in accordance with the above considerations, we are compelled to understand as being general and universal.
47. But when He says, “saving for the cause of fornication,” He has not said of which of them, whether the man or the woman. For not only is it allowed to put away a wife who commits fornication; but whoever puts away that wife even by whom he is himself compelled to commit fornication, puts her away undoubtedly for the cause of fornication. As, for instance, if a wife should compel one to sacrifice to idols, the man who puts away such an one puts her away for the cause of fornication, not only on her part, but on his own also: on her part, because she commits fornication; on his own, that he may not commit fornication. Nothing, however, is more unjust than for a man to put away his wife because of fornication, if he himself also is convicted of committing fornication. For that passage occurs to one: “For wherein you judge another, you condemn yourself; for you that judge do the same things.” And for this reason, whosoever wishes to put away his wife because of fornication, ought first to be cleared of fornication; and a like remark I would make respecting the woman also.
48. But in reference to what He says, “Whosoever shall marry her that is divorced commits adultery,” it may be asked whether she also who is married commits adultery in the same way as he does who marries her. For she also is commanded to remain unmarried, or be reconciled to her husband; but this in the case of her departing from her husband. There is, however, a great difference whether she put away or be put away. For if she put away her husband, and marry another, she seems to have left her former husband from a desire of changing her marriage connection, which is, without doubt, an adulterous thought. But if she be put away by the husband, with whom she desired to be, he indeed who marries her commits adultery, according to the Lord's declaration; but whether she also be involved in a like crime is uncertain—although it is much less easy to discover how, when a man and woman have intercourse one with another with equal consent, one of them should be an adulterer, and the other not. To this is to be added the consideration, that if he commits adultery by marrying her who is divorced from her husband (although she does not put away, but is put away), she causes him to commit adultery, which nevertheless the Lord forbids. And hence we infer that, whether she has been put away, or has put away her husband, it is necessary for her to remain unmarried, or be reconciled to her husband.
49. Again, it is asked whether, if, with a wife's permission, either a barren one, or one who does not wish to submit to intercourse, a man shall take to himself another woman, not another man's wife, nor one separated from her husband, he can do so without being chargeable with fornication? And an example is found in the Old Testament history; but now there are greater precepts which the human race has reached after having passed that stage; and those matters are to be investigated for the purpose of distinguishing the ages of the dispensation of that divine providence which assists the human race in the most orderly way; but not for the purpose of making use of the rules of living. But yet it may be asked whether what the apostle says, “The wife has not power of her own body, but the husband; and likewise also the husband has not power of his own body, but the wife,” can be carried so far, that, with the permission of a wife, who possesses the power over her husband's body, a man can have intercourse with another woman, who is neither another man's wife nor divorced from her husband; but such an opinion is not to be entertained, lest it should seem that a woman also, with her husband's permission, could do such a thing, which the instinctive feeling of every one prevents.
Source: Our Lord's Sermon on the Mount (New Advent)