Unlike in Moses (Deuteronomy 24:1-4), in the New Testament, Jesus forbids breaking up a couple, as He says in Matthew 5:17-18 that He did not come to loosen Moses and the Prophets. Except in the case of fornication.
In the New Testament Jesus expanded adultery beyond physical sex with the woman of another man to include the desire or lust in the heart of the man after a woman, the letting loose of the other spouse (from the man or from the woman) for any reason other than fornication, and the cleaving to a woman who was let loose (fornicator).
So, adultery, in the Bible, is not the same as fornication, since fornication (pornea) is the only cause for divorce and according to 1 Corinthians 5:1.
Thus, adultery is not a cause for divorce based on the understanding that only fornication is the acceptable cause for divorce.
Yet, a woman may not be with two men at the same time.
All the other sexual relationships except sex with the woman of another woman.
1 Corinthians 5:9-11 ASV
(9) I wrote unto you in my epistle to have no company with fornicators;
(10) not at all meaning with the fornicators of this world, or with the covetous and extortioners, or with idolaters; for then must ye needs go out of the world:
(11) but as it is, I wrote unto you not to keep company, if any man that is named a brother be a fornicator, or covetous, or an idolater, or a reviler, or a drunkard, or an extortioner; with such a one no, not to eat.
1 Corinthians 5:12-13 ASV
(12) For what have I to do with judging them that are without? Do not ye judge them that are within?
(13) But them that are without God judgeth. Put away the wicked man from among yourselves.
(Revelation 2:14 ASV)
But I have a few things against thee, because thou hast there some that hold the teaching of Balaam, who taught Balak to cast a stumblingblock before the children of Israel, to eat things sacrificed to idols, and to commit fornication.
1 Thessalonians 4:3 ASV
(3) For this is the will of God, even your sanctification, that ye abstain from fornication;
