Because the girls have been raised in a repressed society and being slutty is something they consider bad.
Being slutty for your beloved one to catch his interest is acceptable as long as it stays in a private setting (and the MC lowered their inhibition with his "Powers").
The fact that Kate and claire accept a 3-some is based on teh Players subtle influence on them and that they are "loveing" each other (as a mother and daugther you Pervs
).
If you follow the NTR route you influence them to be slutty openly and towards everyone, but on the Love route they are focused soley on the MC.
Plus you don't turn them into sex crazed nymphos that mastrubate and drool as soon as they see a dick... you just make them accept their lust instead of supressing it.