You know everyone can be who they want to be.
But, that's a real messed us family for "him" your boyfriend.
Possibly she cheats because she married him but he is interested with men, that' can make someone feel in-adequate.
I suggest he therefore feels in-adequate.
He truly needs help then and you are not qualified to do this but nor should you have to, this is something he has to sort out for himself because he can give or commit to someone else .
Where you aware of this when you went out with him? You say he was "fine" at the beginning which suggests he told you later.
Do you have people / friends / family to talk to about your feelings thoughts and how he is now making you feel?