The flaw in your theory is that the girls actually do get a say as to what goes down. Nina was the one who was pushing for Alan to hook up with Serena. Alan originally didn't want to. Nina has also fooled around briefly with Joe. Bree did fuck other guys. Like I said, her problem was that she only did it around Drew. And we've seen her on multiple occasions put an end to the night when she was done having sex. We've seen Mira with both girls and at least one guy.
I think if anything your theory could potentially apply to Joe, Grisha and Phil since their girlfriends are allowed to do things with other girls but they aren't but even then I think this opinion is a bit of stretch.