>Be coming back to dormroom after class
>Catch roommate about to make out with a friend of ours.
>Both of them are in relationships with other people. Neither of whom go to this school
>I ask them WTF
>They admit it's fucked up, but say they've been stressed and lonely.
>I ask if this is going to happen again.
>They say no
What the hell do I do? I want to believe them but shit. I'm actually friends with my roommate's girlfriend too, but I don't have her number.
Not your problem bro. Butting yourself into other people's business is going to end you up with no friends in a hurry. Either your these two's friend, or your their SO's friend. You can't have it both ways. And honestly, you'll probably end up neither's friend, because nobody wants to be friends with a narc.
How would you feel if your girlfriend or boyfriend was cheating on you, and someone knew but didn't tell you? I'd want to know and because of that I feel obligated to notify anyone when I know for certain their partner is being unfaithful.
Your situation is different, though, because you live with the person you're going to snitch on. You could try talking to them about coming clean on their own but that's still likely to lead to problems.