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.
"Suffering was the only thing that made me feel I was alive,
Thought thats just what it cost to survive in this world,
...now I haven't got time for the pain... " -Carly Simon now 67!