About half a year ago, I explained how to extend marriage to more than 2 people:
Monogamous marriage is the union of two people, so we can say that polyamorous marriage is the union of N people, where N > 2 (and we might bound it to distinguish unions from communes, so we might say that N <= 6). Hence if A is married to B and to C, then B and C must necessarily be married. Divorce occurs when one or more partners withdraw from the union, in which case the withdrawing partners can form their own union or become single. The simplest way to divide property is then in proportion to the number of people in each new, smaller union; custody is more complicated, but it’s already complicated now when only monogamous marriages are recognized.
Of course, this is a restricted situation: it only applies to polyfidelitous groups. But if my reading of the polyamory primers I’ve looked at is accurate, then in the main situation, primary/secondary relationships, there isn’t enough economic interdependence to justify wholesale marriage. It makes sense in that situation to allow certain economic unions, but I don’t think anything like the current situation of marriage is required for secondary relationships. Unfortunately, this leaves open multiple-primary situations.
Back then, I said traditional polygamy would not be covered, because its arrangement is more like this of a star than this of a triangle or a square with diagonals. But now that Jill brought up polygamy again, I’ve thought about this more and realized that there actually is a way of permitting polygamy in a gender-neutral, non-patriarchal way.
Essentially, if John marries Mary and Linda, then Mary and Linda are considered married. That’s perfectly fine, even if they’re straight – after all, some polyfidelitous collectives are straight, so that the males only have sex with the females and vice versa. In terms of legal privileges, the relationship of Mary and Linda isn’t very different from the one either has with John. From what I’ve read about harems, the bonds between the wives are strong enough to justify giving them such rights as to make medical decisions for one another, to inherit one another’s property without paying an inheritance tax, etc.
Of course, the polygamists who want these relationships to be legal don’t want an egalitarian relationship. But the law doesn’t need to account for that any more than it needs to give special status to monogamous D/s relationships. John, Linda, and Mary are free to designate John the head of household and agree that Linda and Mary must obey him, just as they would if John only married Linda.
The other problem, abuse in Mormon marriages, doesn’t need to be treated any differently from in Protestant marriages. The current abuses are independent of whether the arrangement is polygamous or monogamous; they come not from the existence of harems, but from the practice of marrying off girls too young to have legal rights to abusive men. If that’s an argument against legalizing polyamory, then NAMBLA is an argument against legalizing gay marriage.