While I don't want to a) say that it wasn't skeevy for Woody Allen to start a sexual relationship with a woman to whom he was a parental figure when she was a child; b) say that if Woody Allen did sexually molest this other woman when she was a child, that isn't horrific; c) say in any way that someone's adopted child is not their child, I do think
-the taboo against paedophilia is about safe spaces for children.
-the taboo against incest is about propagation of a healthy species (after all, it's a taboo that extends to adulthood and to peer-to-peer blood relationships).
I realise you say "one of the reasons" and specify parent-to-child incest, but I just think that's artificial. The ban on incest is about specific blood relations you shouldn't have sex with. The ban on paedophilia is about protecting children. Parent/child incest is the intersection of those two taboos, but IMO the protecting children bit comes from the ban on paedophilia -- and disappears once the child is in fact an adult.