Incest may be disgusting, but so are a lot of other entirely legal things; you can't legislate morality, though you can certainly legislate resentment and force those you brand "deviants" underground, which often means association with violent criminals, even participation in their nefarious activities.

Necrophilia is a whole other matter, because it's desecration of a corpse who has no say in the matter, which is offensive to the surviving family regardless of how one thinks it impacts the dead. There's a strong argument, however, that there IS an impact on the dead; one horrid little quote on bash.org claims, "it's not rape if she's dead, " but if someone's incapable of giving or withdrawing consent, by most legal standards it IS rape.