Polyamory and the State

Anti-gay marriage activist Stanley Kurtz says of Andrew Sullivan:

In my recent piece, I also noted Sullivan’s failure to take up the real challenge of the slippery slope argument as it relates to polyamory. In fact, I don’t think the word polyamory has ever been formed on Sullivan’s keyboard.

The word “polyamory” has never been formed on my keyboard either, until now.
I’d like to ask Dr. Kurtz, and anyone else, why I’m supposed to worry about this in the first place.
I understand the slippery-slope argument. Gay marriage may lead to legalized polyamory. This argument is intended to freak me out, but it doesn’t.
Let me be clear here. I think people who get involved in polyamorous relationships are making a terrible mistake. I went to college in Eugene, Oregon, and I saw quite a bit of that there. It always ended in disaster; otherwise strong relationships disintegrated with the addition of a third person. Kathe Koja wrote a truly harrowing novel about this called Kink, which reads almost like a horror story. That book all by itself would have been enough to scare me away from polyamory if I hadn’t known better already.
But why is this the state’s business?
The best argument is that a three-parent household is a poor environment for raising children. But what about childless three-way relationships? For whose benefit are they banned?
Besides, I can think of plenty of destructive behavior the state doesn’t regulate, even when it severely affects children. Drinking and smoking, for example, or letting the television babysit the kids all day. You can raise your kids in a wacko cult, or teach them that Jews are Satanic demons that control the Congress. You can deny your children vaccinations and medical treatment for religious or other kooky reasons. It’s best that divorced parents live near each other for the sake of their children, but no one thinks to enforce it.
I think children should be kept far away from television and given books to read instead. I think American children should learn a foreign language when they are pre-school age. And I think they should be kept out of churches until they are old enough to understand what’s going on in there. I don’t think anyone under the age of 20 should have a baby. But I would not dream of legislating any of this stuff, even though I think it would make children better off.
To me it’s obvious that a healthy two-parent household is the best place for a child. But I wouldn’t ban divorce in order to enforce it. Nor would I require single parents to get married. Most people agree with that.
So why is polyamory the state’s business?
I’m not convinced that it isn’t the state’s business. I honestly don’t know. I’m open to persuasion either way. But I really think the burden is on the state to tell us why it should be allowed to micromanage our personal lives in this (and any other) way.
It’s a free country, folks, and not every bad thing is or should be an illegal thing. Even when kids are involved.
Stanley Kurtz wants Andrew Sullivan to address the slippery slope to polyamory. And I want conservatives to tell me why I’m supposed to be afraid of it. And don’t tell me polyamory is not a good idea. I know that well-enough already.