If you were cheated on ..

Would you like to know? 
My friend says everybody cheats or is cheated on at some point. Says it's stupid to confess and also wouldn't like to know if her boyfriend cheated on her. And that if she found out she may forgive him and stay with him but certainly get even and cheat on him too. 
I don't get any of that. I'd like to know so I can leave him. But maybe that's too idealistic. Maybe I'd crumble and stay with him anyway or  maybe it's better not to know.