Some people raise a slippery-slope argument against marriage equality for same-sex couples. "If you argue that same-sex couples have a moral right to equal access to marriage, what moral grounds do you have for denying polygamous couples equal access to marriage?"
There are a number of counter-arguments to that, but the one I will focus is that it is just not possible to grant equal rights to polygamous spouses. Current marriage laws codify a symmetric, reciprocal arrangement and it is not administratively possible to make asymmetrical marriage laws that are fair to everyone.
Who has the right to make medical decisions? What if two spouses wanted to disconnect an incapacitated one from life support and two did not? It would make the Terry Schiavo case look straightforward.
Suppose that Bob and Carol marry. In community property states, Bob gets half of Carol's community property and Carol gets half of Bob's community property. Then Bob marries Alice as well without Carol's consent. Then what happens to community property? Does Bob still get half of Carol's and Alice's, while Carol suddenly, through no choice of her own, now only get a third of Bob's? That doesn't seem fair. Does Alice get nothing? That doesn't seem completely fair either.
Bob and Carol have been married for fifty years and retire. After she retires, Carol marries Alice. Who gets what percentage of Carol's pension when she dies?
Ted and Mabel are both veterans. Ted has one wife, while Mabel has twenty-three husbands. Ted's wife gets various veteran's benefits, including educational benefits and the right to be buried with Ted in a veteran's cemetery. What benefits do Mabel's husbands get? If they each get the same benefits as Ted's spouse, then Mabel's household gets more money from taxpayers than Ted does. If Mabel's husbands each get one-twenty-third of the benefits, then they get less than Ted's wife does. And how would you bury one-twenty-third of each of Mabel's husbands with her?
Alice marries a man from Ghana and sponsors him for citizenship. Jack marries sixteen women from Venezuela. Can he sponsor them all for citizenship? It isn't fair if he can; it isn't fair if he can't.
People are granted immunity from testifying against their spouses and in some cases are not allowed to testify for or against their spouses. Can an entire street gang marry each other to make sure that nobody testifies against anyone else?
If Jane, Jack, and Lisa are all married to each other, then issues involving divorce -- particularly around children -- get exceedingly interesting. Presumably, all three would be legal parents of any children. And if we throw Carol and Bob and Ted and Alice into the mix, the paternity might not be clear. How do you work joint custody among twenty parents? If it's hard enough to collect child support payments from one parent, how difficult will it be to get child support from nineteen?
First, while rights are easy to assign to people, responsibilities are much harder to assign. Far more people will sign up to inherit my assets tax free than will sign up to support me in my old age. I could give Bill Gates the right to make medical decisions on my behalf without his consent; I don't think I could get away with assigning Bill Gates responsibility for my debts.
Second, I am absolutely certain that being able to assign rights to anybody would lead to abuse of the system. I bet it would take less than a day before people started auctioning their spousal health benefits or citizenship rights. I don't think that's what we want either.
You might be able to do something where you have sets of rights and responsibilities that are tied to each other; that you can have my assets when I die only if you also sign up for my debts. Figuring out what to tie to what would be extremely difficult.
The best you could hope for would be "sort of equal", and it would be fiendishly difficult to figure out how to rewrite the legal code to do that. The General Accounting Office found 1049 laws that treated people differently depending on marital status. Each state has several hundred laws. Counties and cities sometimes have laws. Government entities and corporations have regulations. All of those laws assume that a marriage is between two people.
To expand marriage laws to include polygamous relationships would mean going through each and every one of the hundreds of laws. The rights and responsibilities could not be made equal, so every single law would require thought and work and arguments about the most equitable way to handle multiple spouses. It would take years.
By contrast, it only takes replacing the words "husband" and "wife" with "spouse" to end marriage discrimination against gay and lesbian couples. It would be easy.