Thomas Aquinas

But if she does separate, namely, on account of fornication, let her remain unmarried, as long as the husband is alive, because although the marriage is dissolved as to bed and board, not as to bond. Or else be reconciled to her husband, namely, if the husband is not continent. Likewise the husband should not divorce his wife, except on account of fornication. A similar form is kept in regard to the man and to the woman. …

Commentary on 1 Corinthians ↗