JQS posted this in another thread and I thought it really summarized my thoughts about casual sex and FWBs too
"For me there is a big difference between casual sex and sex with a friend.
For me "casual sex" involves sex with someone that I am not particularly close to - someone I meet at a party, have a good time with, but don't necessarily care if we ever see each other or speak again - although I might be up for a second go-round if we happen to run into each other again - I wouldn't necessarily go to any special effort to make that happen.
On the other hand, I don't have any "casual" friendships - by the time I consider someone a friend they are an integral part of my life and my heart (anyone else is an acquaintance). Their needs and feelings are very, very important to me. Which is why I generally don't have more than a few friends at any given time. (I am easily "friend"-saturated). While the sexual nature of a "FWB" situation might be "casual" - the friendship itself never would be - the sex part is a small part of the relationship."