Below is a comment that I left on the Volokh Conspiracy some time ago. Rather than squander perfectly good ramblings there, I have reproduced it here and attached a link.  Th’ Gaussling

Broadly, we have two kinds of marriage in the USA. One is before a god and the other is before the state. Marriage before a god is a supernatural arrangement that is beyond the scope of this letter.

It would seem that the states compelling interest in marriage is mostly confined to the disposition of debts, assets, and minor children during the marriage and in the event the marriage fails. Married partners have an obligation to the welfare of minor children born to them or adopted. Married partners also have a status that allows for decision-making in critical care situations. It seems to be a kind of partnership whereupon responsibility for the secular aspects of married life are defined. After all, the state is called in to make decisions as to the disposition of civil matters in the event of a divorce. Surely the state can clearly define certain basic responsibilities and privileges in advance.

The moral/spiritual aspects of marriage “can” be interpreted as being perpendicular or orthogonal (like the x and y axes in a graph) to the legal dimension of property rights and other secular aspects of married partners. The state is without supernatural powers, thankfully, so it is inherently impotent in the spiritual dimension. If that is the case, and in the absence of a uniform interpretation of supernatural governance, it should be silent on spiritual matters.

The state should have no interest in how married partners conduct their lawful affairs beyond the normal confines of civil and criminal law.

A code defining the responsibilities of married partners in a variety of configurations could be modeled easily. If you accept the premise that secular marriage is confined to the mundane matters that are already contestable in a court, then it is a simple matter to imagine same sex or plural marriages under the same constraints. What is the compelling interest of the state in barring same sex partners from having automatic authority in giving comfort to a dying partner? We already have codes regulating many other kinds of complex relationships between people- corporations, partnerships, LLC’s, government, etc. Minimally, the state should entertain the prospect of recognizing limited entry of some new definitions of marriage to adult parties wanting to be responsible members of society with the rights and responsibilities thereto appertaining.