But they bring up a valid point, because (as far as I remember) so far no other characters (besides F, D, Olivia and Graham) have shown a positive response to the taboo of incest.Pally..... Stop pissin on my dreams my dude.... How dare you bring logic to this XD
So while other characters may already be interest in a relationship with either F or D, and could potentially become interested in a polygamous/sharing relationship with one of them, the biggest hurdle/barrier that needs to be overcome for that relationship to include both F & D is the taboo of incest.