But if the man is in a relationship, and he makes that clear before you sleep with him, please know that his relationship status and, thus, your position in his life will not change unless he wants it to change. If you are willing to sleep with him while he has a girlfriend, he has no reason to change anything because he has the best of both worlds.
He will return to the girlfriend, because initially you have given him permission to do so. In his mind, your participation in the Friends with Benefits arrangement means you will not interfere with what he has going on at home, and you will know your role and keep your mouth shut about it.
Your great sex, pole dance, sexy attire, and late-night romps in exciting places will put a smile on his face, but it will not change your FWB position in his life. Conversely, your tears and threats to leave him or expose your FWB arrangement to his girlfriend will only push him away from you.
You agreed to be the chick on the side when you provided the sexual benefits. There is no promotion to girlfriend or wifey unless that is something that he wants to pursue.
So before you enter into a Friends With Benefits arrangement, ask yourself, “benefits” for whom?