Can't say that I've ever slept with a boyfriend's dad, although there have been a few that I would not have said no to.
But - ask me if I've ever slept with my girlfriend's (college roommate) dad - a big resounding yes. An affair she doens'y know about that started sophomore year. Hot and heavy for a while, then calmed down - now it is an occasional very satisfying (to both of us) romp. Its a long story how it started - might be the germ of another story for Lush ...