Hiya.
Unfaithfulness is not excusable on grounds of being trans. Considering how open and supportive you seem to be, he does not come across as being very grateful.
I would say, like any sex act, if you have a significant aversion to it then don't do it. It would not be right to pressure you into doing it. If it means that you are no longer sexually compatible then it is unfortunate, but ultimately, it is so. Either way though, I would not be able to excuse the unfaithfulness.
Good luck whatever you decide
Akira x