Objection 1. It would seem that, in a case of divorce, husband and wife ought not to be judged on a par with each other. For divorce under the New Law takes the place of the divorce [repudium] recognized by the Old Law (Matthew 5:31-32). Now in the "repudium" husband and wife were not judged on a par with each other, since the husband could put away his wife, but not "vice versa." Therefore neither in divorce ought they to be judged on a par with each other.
Objection 2. Further, it is more opposed to the natural law that a wife have several husbands than that a husband have several wives: wherefore the latter has been sometimes lawful, but the former never. Therefore the wife sins more grievously in adultery than the husband, and consequently they ought not to be judged on a par with each other.
Objection 3. Further, where there is greater injury to one's neighbor, there is a greater sin. Now the adulterous wife does a greater injury to her husband, than does the adulterous husband to his wife, since a wife's adultery involves uncertainty of the offspring, whereas the husband's adultery does not. Therefore the wife's sin is the greater, and so they ought not to be judged on a par with each other.
Objection 4. Further, divorce is prescribed in order to punish the crime of adultery. Now it belongs to the husband who is the head of the wife (1 Corinthians 11:3) to correct his wife, rather than "vice versa." Therefore they should not be judged on a par with each other for the purpose of divorce, but the husband ought to have the preference.
Objection 5.On the contrary, It would seem in this matter the wife ought to have the preference. For the more frail the sinner the more is his sin deserving of pardon. Now there is greater frailty in women than in men, for which reason Chrysostom [Hom. xl in the Opus Imperfectum falsely ascribed to St. John Chrysostom] says that "lust is a passion proper to women," and the Philosopher says (Ethic. vii, 7) that "properly speaking women are not said to be continent on account of their being easily inclined to concupiscence," for neither can dumb animals be continent, because they have nothing to stand in the way of their desires. Therefore women are rather to be spared in the punishment of divorce.
Objection 6. Further, the husband is placed as the head of the woman in order to correct her. Therefore his sin is greater than the woman's and so he should be punished the more.
I answer that, In a case of divorce husband and wife are judged on a par with each other, in the sense that the same things are lawful or unlawful to the one as to the other: but they are not judged on a par with each other in reference to those things, since the reason for divorce is greater in one spouse than in the other, although there is sufficient reason for divorce in both. For divorce is a punishment of adultery, in so far as it is opposed to the marriage goods. Now as regards the good of fidelity to which husband and wife are equally bound towards each other, the adultery of one is as great a sin against marriage as the adultery of the other, and this is in either of them a sufficient reason for divorce. But as regards the good of the offspring the wife's adultery is a greater sin against marriage than the husband's wherefore it is a greater reason for divorce in the wife than in the husband: and thus they are under an equal obligation, but not for equal reasons. Nor is this unjust for on either hand there is sufficient reason for the punishment in question, just as there is in two persons condemned to the punishment of death, although one of them may have sinned more grievously than the other.
Reply to Objection 1. The only reason why divorce was permitted, was to avoid murder. And since there was more danger of this in men than in women, the husband was allowed to put away his wife by a bill of divorce, but not "vice versa."
Reply to Objections 2 and 3. These arguments are based on the fact that in comparison with the good of the offspring there is more reason for divorce in an adulterous wife than in an adulterous husband. It does not follow, however, that they are not judged on a par with each other.
Reply to Objection 4. Although the husband is the head of the wife, he is her pilot as it were, and is no more her judge than she is his. Consequently in matters that have to be submitted to a judge, the husband has no more power over his wife, than she over him.
Reply to Objection 5. In adultery there is the same sinful character as in simple fornication, and something more which aggravates it, namely the lesion to marriage. Accordingly if we consider that which is common to adultery and fornication, the sin of the husband and that of the wife are compared the one to the other as that which exceeds to that which is exceeded, for in women the humors are more abundant, wherefore they are more inclined to be led by their concupiscences, whereas in man there is abundance of heat which excites concupiscence. Simply speaking, however, other things being equal, a man sins more grievously in simple fornication than a woman, because he has more of the good of reason, which prevails over all movements of bodily passions. But as regards the lesion to marriage which adultery adds to fornication and for which reason it is an occasion for divorce, the woman sins more grievously than the man, as appears from what we have said above. And since it is more grievous than simple fornication, it follows that, simply speaking, the adulterous wife sins more grievously than the adulterous husband, other things being equal.
Reply to Objection 6. Although the control which the husband receives over his wife is an aggravating circumstance, nevertheless the sin is yet more aggravated by this circumstance which draws the sin to another species, namely by the lesion to marriage, which lesion becomes a kind of injustice, through the fraudulent substitution of another's child.
Source: Summa Theologica (New Advent)