I think that our marriage laws are still archaic, for one thing. I believe that any group of people who truly love each other should be permitted to legally marry. I favor gay marriage, and I don't see any good reason to outlaw polygamy, or polyandry either.
This throwaway comment also generated a follow-up post, in which Den Beste acknowledged that polygamy might well lead to individual misery, but that anything that doesn't cause society collective harm is none of society's business.
Fair enough--I agree with the generally libertarian principle at work here. But I should point out that nothing is stopping Den Beste from marrying another man, or several other men, or several other men and several women, plus two dogs and a goat (although animal cruelty laws might have something to say about marital activities with the dogs and goat). That is, if he can find someone from the Church of Atheism or whatever organization might bless such a union to do the blessing, no law prevents it. Oh, there are a few places where sodomy, fornication, and adultery are actually illegal, but such laws wouldn't be likely to survive a court challenge. The freedom which Den Beste demands already exists.
What homosexuals and polyamorists can't do is get the State to bless their union with a marriage certificate. And there's nothing fundamentally philosophically wrong with that. For a marriage certificate is not an entitlement or a grant of special rights unavailable to unmarried members of society, it is a contract which grants certain specific rights and imposes certain specific obligations on the persons getting married. It also alters their obligations to and relationship with the State and with others with whom they form contracts. In particular, a marriage certificate alters the relationship of two people with regards to property ownership, and alters their relationship with the state with regards to taxation. From a legal point of view, marriage has nothing at all to do with love, fidelity (except insofar as infidelity is a kind of breach of contract), childrearing, sex, or happiness. It has to do with who inherits what, what the marginal tax rates of the married couple is, and who has the right to make medical decisions for those in a coma.
From this perspective, civil marriage does not exist to unite people who "truly love each other," indeed, it isn't clear how the State would determine such a thing, or why on Earth a libertarian thinker like Den Beste would want it to try. It exists to promote specific public-policy goals, e.g., encourage (but not guarantee) that children will be raised in a two-parent household (which will reduce their tendency to become criminals), promote public health by discouraging disease-spreading promiscuity, and permit individuals to artificially grant the person of their choice status as "next of kin" in favor of blood relatives for a variety of legal purposes.
For my part, I favor gay marriage as I believe it advances the goal of encouraging fidelity and offers considerable benefit to those who want medical and financial decisions to be made by their romantic partner rather than parents or siblings. I see no harm and lots of benefit to both individuals and society in it. However, it is the proponents of gay marriage who have an obligation to argue that creating a new provision in the marriage law benefits society; they cannot simply sit back and chant "equal rights" because marriage isn't a right inherent to our humanity (such as freedom of conscience or self-defense), it's a privilege granted because it's good public policy.
I'm a lot less certain about polygamy; it's hard for me to see the benefit to society or even to individuals. The web of competing property interests seems to be a sure-fire recipe for disaster. How would "joint (several?) tenancy with right of survivorship" apply to 6 lovers? Would 5 of them be required to divorce the sixth, or would a majority vote (3-2) be enough? Could person A marry both person B and person C, without B being married to C? If so, how would the property be divided up in the event of a divorce? Half to A, half to B, half to C?
I don't support prosecuting a guy who has several women living in his home as his "wives," and if that's what's happening, we should put a stop to it. But I can't really imagine a good way to sanction multiple-partner marriages in the same way we could easily sanction gay marriage.
Den Beste is a smart guy; I don't think he thought this all the way through. And of course I could be wrong in believing that no one would be prosecuted for living in a big house and sleeping with everyone there.