Im at a new college and Im playing "grab-ass" with many girls who already have boyfriends. The thing is that I seldomly spend time with other people because most people are either: too kind, boring, nice, swallow, un-intelligent.
I once trapped a girl who made fun of one of my best buddy. I seduced her and made her (saying that you have someone "make" or "do" something is rather a bit too much and exagerated. Id say she willingly did it) leave her current boyfriend so we could hook up. She had been really mean to him and insulted him during the break-up session. When it came to sex, I just told her "no thank you, Id rather masterbate than have sex with you". She was destroyed and powned.
A bit more on topic: As long as you do not lie about your motives you have every rights to expect sex only.