I don’t want to make polygamy legal. God forbid. All I’m saying is that it’s a free country and people can be idiots if they so desire. And just because a pile of people decide to be idiots, that doesn’t mean they should not be allowed to procreate. And they should not be punished for being idiots. Freedom of association, freedom of religion, free speech, etc. The Constitution covers idiots, too.
Actually, events usually do punish people for being idiots. (God is an iron.)
However, the State punishes adults who harm or conspire to harm 3rd parties, including their own children. In these cases, we aren’t talking about the State and individuals. We are talking about the State, adults, and the children they are harming.
Of course, it’s only sex. And only one or two a year. Not immediate, just nice slow, methodical and systematic abuse.