The best way to do what you suggest is to use Nisa's Wicked Perversions and have them all become dolls. Unfortunately, they would all have to live with you in that case. That being said, it allows you to determine whether or not your girls are allowed to do the nasty with anyone else.
Like you, I've had major problems with WW autonomy system. It just doesn't fucking work. I don't remember which mod, whether WW or Nisa's has a "sexually abstinent" trait which I almost always end up giving the male sims in order to prevent just what you said. I don't want NPC's banging my sims! Also, I make sure my sim at least gets a first kiss, and makes them his girlfriend. (I give him the polyamory trait so he can have more than one gf...) This seems to help somewhat in lieu of Nisa's, but it isn't perfect. I feel your pain lol. Perhaps what we really need is a "faithful" trait or something?
Edit: Oh, forgot to mention. If you disable always accept in WW, is helps. But then that creates other issues like your sim not being able to invite others to the mix even if they are your sims GF... Kinda sucks... lol.