Personally, I would be a lot more worried about the fact that your husband cheats on you using hook-up sites than about the specific sex/gender of the people he cheats on you with. Ultimately, he went out of his way to seek out risky sex with other people. That's what would bother me, not the person he chose to cheat with. But that's just my personal feeling.
If you, however, feel that his sexuality (ie being pansexual) is the bigger deal, then even if you forgive him for cheating, I don't think you can expect to have a healthy relationship with him. It isn't fair (on either of you) to stay together if you think his sexuality is abhorrent/offensive/wrong. It would beyond dysfunctional on both sides. You would both be miserable and end up hating each other (if you don't already).
I don't want to leave him
Don't, then. But if you choose to stay with him, clearly you can't expect him to somehow change his sexuality so that he's only turned on by people of a sex/gender/appearance that you personally find acceptable. Regardless of what you think of trans people, you can't just somehow stop your husband finding them sexually attractive.
Setting his sexuality aside - you also cannot somehow go back in time to before he cheated on you. This will never unhappen. So if you can't forgive it, you need to accept that your marriage is over. Personally, I wouldn't forgive someone who cheated on me, especially with someone he actively sought out on a hook-up site - but I also wouldn't continue the relationship.
The only options here are:
a) stay with him, and somehow accept and be at peace with his sexuality and/or the fact that he cheated on you
b) stay with him, and continue to massively resent and be disgusted by him, in the knowledge that this makes you both angry and unhappy
c) leave him
Personally, I don't think options and A and B sound very feasible. But it's up to you.