Yeah, I have to agree with Jason. I have this idealized fantasy about good friends who share casual sex once in a while, but my experience with men and women is that sex changes everything. A lot of it is how I look at it, of course. If I could treat it as casually as some of the people here seem to be able to, there'd be this long list of fuck buddies, one-night stands and broken hearts...
The problem for me with women is that they tend to fall in love with me. Once I fell in love with one, and it was amazing. The sex went to a whole different level, we got married, and I was one supremely happy man.
The problem for me with men is that, while I love getting blow jobs from guys, I am not into giving them myself or doing the other sexual activities guys tend to do together. Additionally, I am always wanting it to be more than just sex with guys, but it doesn't seem to work for me.
I have several guy friends who have wanted to take it to the sexual level for years, but I have always felt we'd not remain friends if that happened. Conversely, I've had some other friends who I've wanted to get more intimate with, but I suspect our friendship has continued, such as it has, because we never did play that sexual card.
I did have three somewhat long term relationships with guys, but none was ever really a good fit. I've also been serviced by a few guys, and while I'd rather masturbate than treat someone as merely a convenient source of release, I'll probably do it again.
All of that is to say that one should take the sexual step with friends very cautiously if it seems nothing more than curiousity or lust.