There are definite parameters (mostly unspoken, but expected to be understood), from the male perspective when it comes to a FWB relationship. One of them, being that there is not supposed to be any pressure or “drama” from the female companion. Most men whom seek the FWB title are not ready for a serious relationship from the onset. The accountability is a factor that men try to avoid.
We also know that achieving a FWB relationship is a rare opportunity. Once gained, we desperately try not to lose it, often times at the expense of the significant other’s feelings. Your description of him as a selfish boy, is more accurate than that of a heartless man.
Men want FWB relationships to purely focus upon receiving physical satisfaction and we ambitiously wish that the female has the same mentality. Emotions and any other connections are added benefits, although the male focus always remains on the physicality. Any attempted shift in this focus may cause mixed results.
I hope this helps.