This source says Friends With Benefits (FWB) isn't about feelings. And so it's not really about "friends" at all -- since friends do have feelings for one another.
I looked at other www.google.com
hits on "Friends With Benefits" and they say stuff like ... "no strings attached" ... "no chocolate, no roses" .... It's all about SEX.
Sex is a powerful human drive, no doubt about it -- but if we want friends, real friends, they aren't going to be FWB. And if we want our friend also to be our lover, in the TRUE sense of that word, there has got to be some real loving with the sexing!