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.
"And God only knows
God makes his plan
The information's unavailable to the mortal man
We're workin' our jobs
Collect our pay
Believe were gliding down the highway
When if fact we're slip slidin' away"Paul Simon