I'm saying NAMBLA and the rest of the "boylovers", believe that children, psychologically, can give informed consent as much as any adult to sexual activity with adults. They believe that those children should be granted the individual rights. And the "zoophiles" say the same thing about animals; animals can show through their behavior whether they like or dislike sexual contact. Both of boylovers and zoophiles argue that the claim that children and animals can't give informed consent is founded in the same irrational taboos from which homophobia springs. Thus, both groups conclude that the government should just butt out when children give their informed to have sex with adults.
So prove them wrong.
Then that's an entirely different question. I am assuming that in our discussion of polygamy, we are speaking of consenting adults. The NAMBLA argument is as outside this discussion's realm as would be the argument of someone who claimed a severely mentally handicapped person could give proper consent, or a discussion of the appropriate age of suffrage, or whatever.