What would you do in a complicated FWB friendship?

i would suggest for you to just offer the "friend" part of you. don't give him the "benefits" since he has another girl, although he says "it's not cheating". that's the only way for you to really know if he's a real friend or if he's just using you... also, try to find other friends to hang out with. that way you won't miss the time you use to spend together as much. you don't have to cut off the friendship completely, but the more space you have in between you two, the better.