I am not OK with legal polygamy, since that involves legal marriage, rather than sex acts. Sex acts with by adults with minors are per se without deemed without adult consent by the minor, and thus are a crime everywhere. Adult consent matters in the law when it comes to sex, and should. Surely you know this.
No, pedophilia is not a crime everywhere. And how can you know if a young girl or boy is or is not capable of experiencing pleasure from such? If everybody has a good time, who are you to judge?
I doubt that there's any anti-pedophilia gene in human beings that would naturally predispose us to regard it as an abomination.
No, our revulsion of it comes from something outside of us, and it is the same that prohibits us from acceptance of polygamy.
That's the moral argument.
Then there's the political argument: polygamy, allowed in any form, will destroy what remains of this country.