I suppose it's just all fantasy fulfillment, as yes, Riona and Camila obviously have plenty of sex with other people, but they offer themselves to the MC "for free" because he's just so much hotter and better than all the rest. Quinn is stone cold with any other guy, but softens up with the MC and only for the MC. In fact, virtually all of the women in BaD are having plenty of sex with other people, or at least aren't virgins. The only exception is Jill, as far as I can tell. Even Maya is having sex with someone else.
But I love that the anti-NTR hysteria is being countered. MMF threesomes aren't really in my fantasy wheelhouse, but I could see that potentially being a possibility in BaD. Derek seems like the obvious partner for such a scheme, but he's such a dufus, I'm not sure he's the right pick. Maybe Jacob or even Tommy?