This has definately happened to me! I was really good friends with this guy and it led to something more (which i never thought would happen in like a million years). But unfortunately it became too wierd and he was getting really attached. I viewed our relationship as being casual and I never really wanted it to be serious, so i ended it. I really shouldn't have started it in the first place, but I definately lost the good friendship we once had. I am still friends with him, but he gets on my nerves after a short period of time because i think he still wants me back. But sex is never platonic, it always means something more.