On the path down the slippery slope, incest is now where gay marriage was when the Supreme Court proclaimed it to be a constitutional right. The same arguments apply. The sheep milling around at the San Diego seaside see no problem with this:

Once morality has been exchanged for political correctness, all of the social mores that have held civilization together for centuries are slated for destruction by depraved social engineers. Normalizing incest will be only a minor skirmish in the Culture War. Progressives have already set their sights on the next hill beyond it: normalizing pedophilia.

On a tip from Blackjack.