sure, i've slept with girls who had been or would be in the gfs of friends. i have not slept with girls while they were actively the gf of a friend cuz that's just bad form and even i have a few scrupples. Of course, there have been times when i've been liberal with the definition of "is" but i'm in pretty good company there.

oh, do 3-somes apply? If yes, then i've clearly slept with the gf of friends but then they were too.

btw, this phenomenon of sleeping with the gf of friends it not at all limited to those of us who are hung so perhaps you can open the question to everyone.