The legal logic of polyamorous marriage
Some people hope (or fear) that same-sex marriage will pave the way for the legal recognition of polyamorous marriages. Some advocates of legal polyamory argue that restricting marriage to duos is every bit as arbitrary and unfair restricting marriage to partners of the opposite sex.
I'm in favor of social acceptance and legal recognition for n-tuples (like couples, except n>2). However, there's no easy way fit the diverse and complex poly-relationships into a convenient package deal of rights and responsibilities comparable to civil marriage. If we want to create legal recognition for these relationships, we are going to have to self-consciously build alternative institutions. This won't just fall into place like gay marriage.
By recognizing gay marriage, we're removing one arbitrary admission criterion without modifying any of the legal ground rules. For example, allowing SSM wouldn't change the rules about community property, inheritance, second-partner adoption, or spousal privilege.
Civil marriage is supposed to make things clearer and easier. It's a legal combination plate, an easy way to designate one person as your next-of-kin, the co-owner of your property, your health care proxy, etc. A two-way marriage is a neat swap: I'm you're go-to person, you're mine. If you add more people into this relationship, questions that were simple become much more complicated.
Consider the issue of health care proxies. Suppose Bob and Ted are married. Bob is Ted's proxy, Ted is Bob's proxy. If either partner gets sick, his husband is designated to step in and make health care decisions on his behalf.
Now suppose that Bob, Ted, and Carrol are in a three-way marriage. Who is Bob's health care proxy? Obviously, that's Bob's decision. He might choose Ted, or Carrol. Or, he might want to name his two spouses as co-equals. Perhaps he'd prefer a line of succession in which Carrol is in charge unless something happens to Ted, or vice versa. Ted and Carroll are also going to have to make their own decisions about proxies. If they're smart, everyone is going to legally register these preferences in order to ensure that they will be respected. This is hardly an insurmountable problem, but marriage doesn't seem to be doing anything to simplify the situation.
Other legal systems have been dealing with multi-marriages for thousands of years. However, it's usually men who have the right to take multiple wives, but not vice versa. A Mormon patriarch may have several wives, but his wives can't take another husband. Modern poly marriage would have to give men and women equal rights (including the right to SSMs). Things could get very complicated.
Monogamous marriage is a symmetric relation: If I am married to you, you are by definition married to me. So far, we haven't made a cultural decision about whether the marriage relationship is transitive because we limit marriages to two people. Under legal polyamory, if I'm married to you, and you're married to someone else, am I automatically married to that person?
The eminently logical Alon Levy suggests that treating the marriage relation as necessarily transitive would make it easier to resolve property disputes and other legal issues. However, as Alon notes, this legal model isn't a good fit for many real-life poly relationships in which A and C have not made any commitments to each other, even though they are both committed to B.
Suppose A is married to B, and D, who's partnered with A and nobody else. Then D divorces A. What percentage of A's property does D get in the divorce settlement? Keep in mind that A's divorce settlement cost B because she holds property with A, and if B loses money, C loses out because B and C are married.