I'm going to expose my roommate and his gf
What exposure are we talking about?
He is cheating and exposing him is the only way for him to stay monogamous with his gf.
Him cheating on his girlfriend is wrong, but I think it would be better to have a talk with him instead of exposing his behavior to everyone since it will embarrass him and his girlfriend. Talk to him and tell him that he needs to tell his girlfriend about his actions.
I still want to expose them.
I understand that you want to expose him, but think of the pain and embarrassment it could cause to him and his girlfriend. Things like this should be kept private between just the people it's relevant to. Talk with him and let him know that he should discuss his cheating with his girlfriend.