Thanks for an excellent post! I found this helpful too:
4. Divorce
Traditionally, there are two grounds for divorce: infidelity (Mt 5:32; 19:19) and desertion (1 Cor 7:15). One preliminary question is why the Matthew version, with its exemptive clause, is more liberal than the Markan version (10:11-12), on the one hand, but less liberal than the Pauline discussion, which “adds” a second ground?
Of course, any answer is bound to be somewhat conjectural, but the following may be suggested. It may be…