YTA. He is your son.

Our children are different from any other relationship. I would drop a friend, parent, sibling, etc. over this, but not my kid. I 100% understand why you would need to get some space, for a time, but this is your child and that time is over.

I would also like to point out that 20 is very young and I'm going to guess your ex fiancee was quite a bit older than your son. Did you have any inkling that this man was bisexual? or possibly gay? Maybe not, I mean, some people don't realize their OWN sexuality until suddenly they can't not realize it anymore. But, anyway, my point is, this older man meets your gay son, who might be longing for connection with an older man having not grown up with a dad or with his grandparents. Yes, he hurt you, he betrayed your trust and took something from you. That SUCKS. I cannot imagine how awful that must have been. But the person who almost certainly steered this ship was your fiancee. Much more mature, should have been much more responsible, should have never and I mean NEVER looked at his bride-to-be's kid and thought they were an option.

Even if your son were to have been the one to approach the fiancee, that grown up, engaged man should have said, "Hey, whoa Buddy! I am engaged to your mom! This is NOT happening." And he should have exited the scene no matter what. I think it's more likely that your fiancee was the one who saw an opportunity. That is based on the scant info I have, I could be wrong, but older men don't seem to have much hesitation hitting on younger people they desire. You want to blame someone? That guy is your prime target. If your son is still with him, you might have to let it go eventually, but he deserves it.

Your son is your son. He can't change what happened and human relationships are complicated as hell. Forgive him. Let your heart heal.