Thanks for the update Nyjah. I assume you've tried saying no and she just overwhelms you either with force or seduction. It may be that you need to find a new place to live, away from this roommate and his nymphomaniac girlfriend.
A guy who enjoys seeing his girlfriend hop on his roommate against his will, is not much of a friend. A girl who gets off on forcing guys into sex is not much of a girlfriend. I say ditch these two as soon as you can.
Seems I've got to have a change of scene
Every night I have the strangest dreams
Imprisoned by the way it could have been
Left here on my own or so it seems
I've got to leave before I start to screamJoe Cocker