Marriage

As religious definitions are laid out in ancient texts, I'm specifically discussing the government institution here. It's not the place of the state to tell a religious organization which ceremonies they are to perform, but the other side of that coin is that the legal definition of marriage ought to be based on facts and reason (as should any other legal policy).

The first amendment prevents the government from establishing marriage based on biblical prescriptions. And rules based on our sexual or reproductive habits would be an affront to personal privacy. As a result, from the standpoint of the government, marriage is little more than a statement of financial partnership; taxes can be filed jointly, money can be easily inherited, and insurance can be joined. Marriage also allows sharing of privileged information such as medical documents even when an individual is incapacitated.

When viewed this way it's obvious that gay marriage is just as valid as straight marriage. But it's not obvious at all that a marriage must be a contract between exactly two people.

It's clear that we can't go back to polygamous times - one man owning several wives - but I would contend that the problem with that is the gender asymmetry rather than the multi-adult household. There are Americans who choose to engage in non-binary long term romantic relationships. It's a small minority, of course, but so is homosexuality. Only a few percent of Americans are gay. And as we are learning in the marriage equality movement, allowing people who love one another to get married does not negatively affect anyone.

The only possible downside of group marriages that I have been able to come up with is that of tax evasion. A super-wealthy individual could in principle acquire dozens of dependents through a massive group marriage, thereby dodging their tax responsibilities. However, this right already exists, more or less, through the formation of a corporation or LLC.