This specific variation on currently popular ideas became defined during a conversation with a good friend of mine.
Let's get this whole marriage controversy about who can marry whom, and nullify it by removing 'marriage' from being a legal term at all, can creating a better separation of church & state.
Instead, domestic partnerships will become the legal standard, and the only standard that matters for any and all legal and/or tax concerns. Those who are already married automatically become Domestic partners, and any further marriages performed by people the government recognizes as credible, and assuming appropriate paper work is filled out and fees paid, will also mark a domestic partnership.
If your church only recognizes marriages between a man and a woman, then no same sex marriages are performed at your church or by your minister. And if the church down the street does recognize same sex marriages, they can perform them. And if you aren't religious, it doesn't matter.
My proposal goes a step further than merely same-sex marriages too. Though I would have the laws state that any individual can only be involved in one domestic partnership contract at a time, I would not limit a given contract to just 2 people. So yes, 3 or more people could get married together, but before to many men go off dreaming of having multiple lady partners... remember, they are partners to each other too, and have every right to divorce your sorry ass, and in a communal property state, that'd only leave you with with 1/3 (or less, if there are more than 2 women involved) the total value of the partnership. While they remain together as partners.
Also, the details of a partnership can be arranged in the domestic partnership contract, everything from pre-agreeing that one partner is supplying the majority of the financial support, to making it part of the contract that sexual fidelity is not part of the agreement (also known as an 'open marriage')
Really, this is all about letting people customize and declare how they want to live with each other, rather than limiting them to the few options that are recognized today (which, on most paper work, is single, married, or used-to-be-married.) And the government can stay out of it for the most part then.