Nope he doesn't cheat. To top it off and to rub more on sensei's guilt, he is genuinely a good person through and through, till the end. And that also contributes to their weird relationship, that's all I can say.
I think this story is deliberately written that way so as to highlight sensei's feelings of how she is truly a trash teacher who abandons moral codes to follow her desire for Rin. The kind of story where mc did bad deeds with barely anything that sugar coats the deeds, but the writting is wonderful enough to make the characters likable, which I give lots of props for.