Joey: ...You've left it too long, and now you're in the Friend Zone.
Ross: Oh no. I'm not in the Zone.
Joey: Ross, you're mayor of the Zone!
...from the sitcom Friends; unless I'm very much mistaken, the episode The One With The Blackout.
It is possible to salvage a romantic or sexual relationship from one that has become merely a good friendship: it doesn't happen often, but it can. It's often counterproductive to do so, however: a good friendship can be a million times more rewarding than a fling.
To underline my point, just look at what happened to Ross and Rachel in the seasons of Friends following the above-quoted episode. I stopped watching Friends when the Friends stopped being Friends.