Where is the victim? If there is no victim, there is no crime. No crime, then no government involvement is required.
Incest is nasty, but who gets hurt? Nobody. No government required.
Polygamy...? Arguably the husband of multiple wives could be considered the victime, but hey, the schmuck did it to himself. What was he thinking!?!?! No government involvement needed.
Pedophilia has a victim: the child. There should be laws. Catch a baby-raper, take him out and whack his junk. Put him in prison and let the inmates know what he did. Nature will take care of it.