Government should recognize and enforce "domestic partnerships" of any two (or more) legal adults of sound mind who want to enter into such partnerships. They should go to lawyers and have those papers drawn up according to state requirements. It wouldn't take more than a few weeks for the legal industry to get that worked out into standard boilerplate equal to most of the current implied rights and responsibilities of marriage. The contract would also include agreements for ending the partnership, if that comes to past. But it would not be called "marriage." Businesses and commercial entities would abide by the parameters of the contract, as happens today when businesses do business with partnerships.
"Marriage" should be taken out of the civil lexicon and reserved for religions and other social relationships. If people want to be regarded as "husband and wife" or "wife and wife" or "husband and husband" or "husband and wife and wife" or whatever, then they would go to the clergycritter of their choice and get that pronouncement done. The government would not recognize those relationships any way, but presumably their personal friends and acquaintances would.
If someone cares about both realms, they'd get both a government-enforced domestic partnership contract and a clergy-certified marriage.