I think most people just assume (because we've been fed this bullshit) that when sex happens between friends, it "ruins" the friendship. So, even if they want to, they don't go there for fear of losing a friend.
I fucked one of my neighbors once (or twice? can't remember). We had had a nice, relaxed friendship. He lived upstairs from me in my little apartment building, and we went and did things together frequently. After we got it on, he was all weird and avoided me. Eventually he moved to Chicago, and I never heard from him again. I always wondered why he couldn't handle it, as I had never placed any expectation n him, and enjoyed his company, but... that's how it went.