I'm a female, early 20's, and share a 2-bedroom apartment with another girl.
A couple of months ago we went to a party. She went with her boyfriend. At the party my roommate started drinking a lot. I had always thought her boyfriend was very attractive, and I decided to take advantage of the situation.
I stayed sober and started flirting with her bf. In the meantime, I made sure she always had a drink in her hand. When she got to the point where she was sitting on the floor in the corner I went with her boyfriend to his car. We were kissing and he had his hands under my sweater. He wanted to have sex (and so did I), so I suggested we take my roommate home, give her a sleeping pill to make sure she remains asleep, and he could join me in my bed.
And that's what we did. We took my drunken roommate (his girlfriend) home, gave her a sleeping pill, and he and I spent the night having sex. Next morning he left before she awakened, and she was never the wiser.
I will note that he and I still have sex when she isn't around. He likes the fact that I will go bareback with him (she makes him wear a condom), and apparently I give a better bj than she does. All-in-all, very satisfying for both of us.