There are very, very few circumstances where this would be a good idea. The ONLY time it would be worth a shot is if a) you wanted to maybe date (still dicey, and not the case here), or b) if you both are very open and honest and easygoing people that have managed "friends with benefits" situations before with other people.
If neither of those applies, then it is ABSOLUTELY a bad idea. If one or both of those apply, then it is just *probably* a bad idea. I guess what I'm trying to say is that you really shouldn't go through with it.
Sex is an extremely emotional occurrence. This can cause people to realize feelings which they didn't think they had. Then, if both people aren't experiencing the same feelings, things can get awkward. Additionally, even if sex doesn't create those feelings, there is still the unusual part of learning how to act around each other: how will you know when you want to have sex versus when you just want to hang out as friends? Will one of you get mad if they want to have sex that night and you don't? Will either of you get jealous if the other person has sex with someone else or acts interested in anyone else?
I think there are just too many possible pitfalls to try it, especially if you don't actually want it to turn into dating.