Two weeks ago, I filed a change in easement for irrigation between myself and my neighbor. We are the only two parties involved and we agreed on the change and had it notarized. The county clerk registers that agreement.
One function of the government is to be the holder of records. Without that no one could really prove who owns what or what agreements or contracts are official. This basic idea needs to be followed with civil contracts between people. Any two people should be able to write up a contract as to how they will share their property and what powers of attorney each one would have over the other. The state, as with any other agreement, simply registers it.
It is the function of religion to give that agreement a sense of holiness and sanctification. All people then have a civil union, those who wish then may take it to a member of their clergy to have it sanctified. That, according to the constitution and basic fairness, is how "marriage" should be treated.