So a random joins a forum and this is his first post. Then, a matter of hours later, the FWB in question comes on and posts... right. As if he would have gone to all the trouble of a) telling you about this, and b) directing you here to talk about your bullshit situation, rather than just ASK YOU at that time.
also, you're a FRIEND WITH BENEFITS. This term is used exclusively to show "we fuck, but there is no real relationship/it's NSA". You have no reason to complain. If you want him to stay with you, ask him out already. Otherwise, don't bitch.