Objection 1. It would seem unlawful for a husband to put away his wife on account of fornication. For we must not return evil for evil. But the husband, by putting away his wife on account of fornication, seemingly returns evil for evil. Therefore this is not lawful.
Objection 2. Further, the sin is greater if both commit fornication, than if one only commits it. But if both commit fornication, they cannot be divorced on that account. Neither therefore can they be, if only one commits fornication.
Objection 3. Further, spiritual fornication and certain other sins are more grievous than carnal fornication. But separation from bed cannot be motived by those sins. Neither therefore can it be done on account of fornication.
Objection 4. Further, the unnatural vice is further removed from the marriage goods than fornication is, the manner of which is natural. Therefore it ought to have been a cause of separation rather than fornication.
On the contrary, are the words of Matthew 5:32.
Further, one is not bound to keep faith with one who breaks his faith. But a spouse by fornication breaks the faith due to the other spouse. Therefore one can put the other away on account of fornication.
I answer that, Our Lord permitted a man to put away his wife on account of fornication, in punishment of the unfaithful party and in favor of the faithful party, so that the latter is not bound to marital intercourse with the unfaithful one. There are however seven cases to be excepted in which it is not lawful to put away a wife who has committed fornication, when either the wife is not to be blamed, or both parties are equally blameworthy. The first is if the husband also has committed fornication; the second is if he has prostituted his wife; the third is if the wife, believing her husband dead on account of his long absence, has married again; the fourth is if another man has fraudulently impersonated her husband in the marriage-bed; the fifth is if she be overcome by force; the sixth is if he has been reconciled to her by having carnal intercourse with her after she has committed adultery; the seventh is if both having been married in the state of unbelief, the husband has given his wife a bill of divorce and she has married again; for then if both be converted the husband is bound to receive her back again.
Reply to Objection 1. A husband sins if through vindictive anger he puts away his wife who has committed fornication, but he does not sin if he does so in order to avoid losing his good name, lest he seem to share in her guilt, or in order to correct his wife's sin, or in order to avoid the uncertainty of her offspring.
Reply to Objection 2. Divorce on account of fornication is effected by the one accusing the other. And since no one can accuse who is guilty of the same crime, a divorce cannot be pronounced when both have committed fornication, although marriage is more sinned against when both are guilty of fornication that when only one is.
Reply to Objection 3. Fornication is directly opposed to the good of marriage, since by it the certainty of offspring is destroyed, faith is broken, and marriage ceases to have its signification when the body of one spouse is given to several others. Wherefore other sins, though perhaps they be more grievous than fornication, are not motives for a divorce. Since, however, unbelief which is called spiritual fornication, is also opposed to the good of marriage consisting in the rearing of the offspring to the worship of God, it is also a motive for divorce, yet not in the same way as bodily fornication. Because one may take steps for procuring a divorce on account of one act of carnal fornication, not, however, on account of one act of unbelief, but on account of inveterate unbelief which is a proof of obstinacy wherein unbelief is perfected.
Reply to Objection 4. Steps may be taken to procure a divorce on account also of the unnatural vice: but this is not mentioned in the same way, both because it is an unmentionable passion, and because it does not so affect the certainty of offspring.
Source: Summa Theologica (New Advent)