ESV 15 But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called you [2] [us] to peace.

It appears that if the unbelieving spouse initiates divorce, or if there the unbelieving spouse is engaged in pornaia, or will not live peaceably with the believing spouse, there is cause for divorce.

In the case of the situation of pornaia,

1. porneia is used (a) of "illicit sexual intercourse," in John 8:41; Acts 15:20, 29; 21:25; 1 Cor 5:1; 6:13, 18; 2 Cor. 12:21; Gal. 5:19; Eph. 5:3; Col. 3:5; 1 Thess 4:3; Rev. 2:21; 9:21; in the plural in 1 Cor.7:2; in Matt. 5:32 and 19:9 it stands for, or includes, adultery; it is distinguished from it in 15:19 and Mark 7:21; (b) metaphorically, of "the association of pagan idolatry with doctrines of, and adherance to, the Christian faith", Rev. 14:8; 17:2, 4; 18:3, 19:2; some suggest this as the sense in 2:21.

and;
My MacArthur Study Bible footnote for that passage says this:
sexual immorality. This is a term that encompasses all sorts of sexual sins. Both here and in 5:32, Jesus includes this "exception clause," clearly permitting the innocent party in such a divorce to remarry without incurring the stigma of one who "commits adultery."

Hope that helps.

The Chestnut Mare