I am a big fan of legalizing stuff. I'm a social libertarian when it comes to adults. I've always thought laws should protect people from other people (i.e. no murder) and protect children and the mentally ill, but if you're of sound mind and body and you want to become a crackhead, I'm not about to stop you, unless I actually care about you personally.
But there's some things that are tricky.
Group marriage is a good example. I'm calling it group marriage because the term "polygamy" is used colloquially to refer to "religious-based polygyny" but "group marriage" both includes traditional polygamy and those weirdos where there's three men and four women and they all love each other.
Now, I think people should be allowed to marry how many people they want. And yet. There should be restrictions. If group marriage was legalized, you would have to say:
-All members would have to be over 18. This applies even if people can marry their first partner at a younger age, like how in Utah it's 14. Teenagers can be a lot more impressionable than adults and you want to hold off that sort of thing as long as possible. They need to be out from under their parent's control, and if my other idea of a national service was enacted, wait until they get out of the national service, when they're 20 or so. The longer they are out from under their parent's immediate control before the marriage, the better.
-All previous spouses must agree. Let's say a man decides he wants to marry his mistress, but he and his first wife don't want to divorce because of the kids, and his first wife doesn't approve. He can't just marry the second woman. The first wife has to give her consent before a judge and that judge has to make an effort to make sure the first wife was in no way coerced. The second wife would also have to be doing this of her own volition (and not under parental control or anything) and be fully aware of the man's situation. I'm thinking waiting periods and consent forms and opt-out clauses and everything.
-All partners have to have regular access to the mainstream community and the right to file a divorce if they so chose.
-You have to figure out certain logistics. Like what happens in case of a divorce? I say if one woman divorces her husband, who has three other wives, then she should get MAXIMUM 1/5th of his money and belongings. He should still have to pay normal child support though (and I favor abolishing alimony).
-Another logistic would be who makes the decisions? Let's say you're in a polyamorous marriage and one of your two husbands is on life support, but your third wife is radically anti-pulling the plug and your first wife thinks it's time. I say that unless the husband writes a living will labeling one of the partners as the decider it should be chosen by vote.
But there are other issues with group marriage, like the toll it takes on a society. Look at the Muslims, who have created an underclass of unmarried, very frustrated men who blow up buildings for God. Now, let's face it. There is no way that legalizing group marriage in America will make it mainstream, not with the above restrictions. There's too many Christians, too many empowered women, and too many educated men who understand the flaws in that system. You would have three groups who try it: Fundamentalist Mormons, who are doing it anyway and at least the prospect of making it legally binding and giving them validity would (potentially) make them come into the open and thus help us regulate the negative sides to it-- plus most of their practices would be illegal under my system anyway; very few Fundamentalist Muslims, mostly just-off-the-boat immigrants-- Westernized Muslims don't do the plural marriage thing; and nutty polyamorous people who are a tiny minority AND tend to be educated enough that people leave when they figure out that they don't like it.
If we make it seem to the religious polygamists that we are more accepting of their lifestyle, they will be a bit more trusting of us. Then we can start educating their children.* Once you educate the younger generation on your own you can start to empower the young women to protest, encourage the young men to see the problems in the system, and over many generations of Westernization the ideas will lose popularity.
Laws don't really change society, education for children changes society, and much faster.
*In an ideal world you would also say that no person can have more than two biological children. That would basically ELIMINATE religious-based group marriage, because those men don't want to support a wife who can't provide offspring. But that's unlikely at best.