Get Your Roommate Drunk: Shouldn't be hard since your roommate is a binge drinking, whore loving douche bag who doesn't know how to appreciate the only girl who is oblivious enough to love him. Just make sure he passes out in the bathroom so he doesn't wake up to you 69ing his woman.
Make Him Look Like A Douche Bag: Also, shouldn't be hard. He is a douche bag. Try doing something like "accidentally" opening up the porn file on his computer. Or play a Creed song from his i-tunes. "He loves Scott Stapp." You will say, "But you must already know that."
Listen To Hootie and The Blowfish's "Hold My Hand": Show her something from your music library. Show her you're both romantic and old school with this classic by Hootie. This is the time to sit down on the futon, yawn emphatically and rest your head on her shoulder.
Hold Her Hand When Hootie says the line, "Hold my hand." : In the second chorus. You guys are in the moment, you're both buzzed. Hand holding is totally innocent. At this point she is second guessing her relationship anyway; you're just helping her decide.
69 Your Roommate's Woman: By the time Hootie strums the last chord you will be engaged in a very heavy make out session with your roommate's girlfriend. You're both turned on by the prospect of being caught, but you lock the door anyway. Clothes flee your bodies, the futon unfolds, and the wild fornication with the forbidden woman ensues.
Stick around: You'll be in your room all year, so you can expect to enjoy a night of amazing roommate's girlfriend sex about every two to three weeks. The best part is that she's already in a relationship. Your douchey roommate is the one who has to deal with that crap, you just get the free sex.