Suppose you got the government completely out of marriage. What then? Can you replace marriage with simple contracts?

People can enter into all kinds of contracts. But those contracts rarely specify every possible contingency. Thus,the state offers default rules. Marriage is no different. Most people want their spouse to inherit most of their estate, make medical decisions, have a cause of action for wrongful death, etc. Hence: civil marriage, which comes complete with a set of default rules which accord broadly with the wishes of married couples all over.

You could write a contract which set up those rights, without calling it a marriage, but:

It's reasonable to assume that the government is capable of enforcing such a contract. There is, or should be (in order for enforcement to work), a place for the government to register such contracts, so it (and you) knows who is contracted to whom, the way it registers land titles. If you're entering in to such a contract, it would be good to know that, in the event of the person to whom you're contracted winding up in the hospital, you are not going to be surprised by some other person who was also so contracted without your knowing. So government is not out of marriage completely.

A contract of this type is a very serious deal. Hypothetical: A rich elderly man hires a housekeeper, and eventually dies of old age. The housekeeper then shows up with a piece of paper with his and her names to a contract like the above, and she is therefore the next of kin, and inherits the man's millions. The kids say "Are you nuts?"

Because of this the government will have reasons to demand certain standards in signing the contracts, similar to the two witnesses required for wills. This may include requiring witnesses, and perhaps requiring the contract to be signed in front of a judge. This is so that, if the validity of a contract is disputed, there's more to go on than the word of just one interested party. So there's a place for government again.

We now have the government enforcing contracts, and regulating under what conditions the contracts will be signed. If the government is going to regulate under which conditions the contract will be signed, then it has to specify what's different about a contract that requires two witnesses and a judge from a contract that doesn't. Once we've got to the stage of government involvement in enforcing such a contract, and in registering who is who's next-of-kin, and in enforcing certain standards to check that the people involved actually both consented, what's wrong about the government supplying a standard contract?

And, once we have the government supplying a standard contract, perhaps we could permit the government to say that if two people have signed this contract, they are married?

Introduce large-scale polygyny or polyandry, and you have a problem, a large number of "spare" un-partnerable people (the FLDS "lost boys", for example), primarily young men. These young men will be awfully angry. If history is any guide, mobs of pissed off young men rarely have a soothing effect on the society at large.

Some legal questions:

• Who would retain custody of children at the end of a plural union? The biological parents only? Would co-spouses have visitation rights?
• Could a group of many people obtain testimonial privilege in court (the right not to testify against one's spouse) simply by entering into a giant plural union?
• Further on the above. Since polygamy will presumably come after gay marriage is legal, what is to stop the establishment of criminal gangs who intermarry to escape testimony against their spouses?
• How would we tax members of plural unions? (If you think the "marriage penalty" is complicated now....)
• If four members of a five-member plural union died, would the remaining survivor be eligible to retain all their Social Security benefits or work pensions? Where would we draw the line?
• Could I marry ten foreign nationals and bestow citizenship via marriage on all of them?
• Surviving spouses are entitled to social security survivor benefits on the theory that a woman who has foregone employment in favor of a domestic life is entitled not to starve after the death of her husband. A pension sufficient for one becomes insufficient when split 10 ways.

• You’re a woman married to a man who, once polygamy is declared legal, elopes with another woman. This enrages you and you decide to divorce him. Before his second marriage you were entitled to half his estate. How has this changed?
• You’re a man without a living will who is married to an even number of women, half of whom want to keep you on life support, half of whom want to pull the plug. Who breaks the tie?
• While driving your car you run over and kill a man with 20 wives. After the wrongful death class action is approved, how is loss of consortium quantified? Is it loss of consortium x 20, or does each wife get 1/20th of the value of the husband’s consortium?

It's not that answers to these questions can't be crafted, it's that a court could not simply declare a right to polygamy without running into a thicket of practical problems. Polygamy would require changes to basically every aspect of marriage under the law -- a hypothetical bill legalizing polygamy would have to be hundreds of pages long. These are not, of course, the only questions that would have to be answered.