I'm Not Going to Play Basket Ball, if Two People Want to Be Together, Then Who Am I?

A female friend has had a huge crush on a man in her graduate school programme. When she confessed her feelings to him several months ago, he told her he wasn't interested in anything more than friendship. A few months later, she went on an inernet date to get her mind off this guy. The next night, i was out with a group of friends that included my friend's crush. One thing led to another, and we ended up having incredible sex. My friend later texted me and asked for space from our friendship. Is it wrong to date or sleep with some one your mutual friend has feelings for, even if you know nothing will happen between them? Signed, the heart