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.
Can't be bothered with sorrow
And I can't be bothered with hate, no, no
I'm using up the time but feeling fine every day
That's why I'm telling you
I just want to celebrate another day of livin'Rare Earth