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.
"In the burning Heart, Just about to burst
There's a quest for answers, An unquenchable thirst
In the darkest night, Rising like a spire
In the burning heart, The unmistakable fire"Survivor