^^^ You'd be a good **** buddy but a lousy relationship partner by the sounds of things. In fact you and the Op's bf's FWB would probably be perfect for one another. People who don't have any personal or relationship boundaries and who cannot emphatize with the feelings of their partner (as in how they would feel with them hanging out one-on-one with a **** buddy) don't make good life partners. It's as simple as that. You cannot be in a relationship and still act single with members of the opposite sex. It's against human nature and the possessive way humans love one another.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion