I think it's eminently reasonable that, if you're going to have some character development of the piece of crap cheating boyfriend, that he would have a rationalization for it. There are lots of people who cheat, and most of them don't want to consider themselves worthless wastes of space, so they'll have rationalizations. So that's just fine, very appropriate.
Except I'm really not interested in having character development for the piece of crap cheating boyfriend.