Augustine of Hippo

(ubi sup.) The Apostle has fixed the limit here, requiring her to abstain from a fresh marriage as long as her husband lives. After his death he allows her to marry. But if the woman may not marry while her former husband is alive, much less may she yield herself to unlawful indulgences. But this command of the Lord, forbidding to put away a wife, is not broken by him who lives with her not carnally but spiritually, in that more blessed wedlock of those that keep themselves chaste. A question also here arises as to what is that fornication which the Lord allows as a cause of divorce; whether carnal sin, or, according to the Scripture use of the word, any unlawful passion, as idolatry, avarice, in short all transgression of the Law by forbidden desires. For if the Apostle permits the divorce of a wife if she be unbelieving, (though indeed it is better not to put her away,) and the Lord forbids any divorce but for the cause of fornication, unbelief even must be fornication. And if unbelief be fornication, and idolatry unbelief, and covetousness idolatry, it is not to be doubted that covetousness is fornication. And if covetousness be fornication, who may say of any kind of unlawful desire that it is not a kind of fornication?