I was just the "other man" by having sex with my ex who currently has a boyfriend.
What's the right thing to do now?
Send the boyfriend a text with a picture of his girl nude or something. Say she's cheating and that you bear no ill will. Sympathize with him.
Don't leave some poor dude with a slut, though.
If I were the guy I'd like you to tell me. I'd be grateful, regardless of who you are she's the one cheating. That it happened doesn't matter it's that it could happen. But not every guy is like me.