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.
"Listen as your day unfolds
Challenge what the future holds
Try and keep your head up to the sky
Lovers, they may cause you tears
Go ahead release your fears
Stand up and be counted
Don't be ashamed to cry " -Des'ree