Get rid of marriage from the law entirely. Marriage could be expressed in a standard contract that gives the sorts of rights the it currently carries. The state shouldn't have any say in who enters this sort of agreement.
We are a species that likes to pair-bond; we consider it a greater hardship to separate members of a pair than members of a looser association like friendship. The legislation reflects that. Social engineering via tax breaks are a dumb idea, but deleting the concept of marriage from law entirely is impractical and probably undesirable.
If you are a new kind of troll, I salute you.
I basically am in favor of religions being able to define "marriage" however they want (and am confident there would be a variant, if not primary variant, of every major religion which supported and another which denounced gay marriage).
The state should do something else, for everyone, with all rights handled through the state's thing.
The quick hack is to just require all new marriages also be civil unions, and not recognize future marriage otherwise. Civil union should get every current private and government benefit of marriage. Strictly religious benefits from private entities could potentially be withdrawn from civil unions (I honestly have no idea what those would be).
I have a lot more faith in a judicial solution than a legislative one. March should be interesting.
The only rights of marriage that cannot be conferred by contract are the parts that fall into the "social engineering via tax breaks" (or redistribution) category.
There's other rights, like hospital visitation, legal indemnification, etc. that also don't fall into that category.
You are correct about immigration and legal indemnification, though I question the social utility of both those benefits.
For example, my earnings go into a account I hold jointly with my wife. If marriage carried no special privileges, then we would need to keep track of how much of the income she "received" from me, to pay gift taxes. "Ok, honey, we ordered a $20 pizza, and you ate four slices, so I guess that means you received $8 worth of income..." Fortunately, we are spared this insanity, because gifts to spouses are tax exempt. This allows us to share our money like we share our lives.
Spousal green cards, spousal privilege in court, and Social Security survivors benefits are other examples of privileges that cannot be granted through private contracts, but that require the government's involvement.
Would that be a general right - any US citizen could grant permanent residency to anybody else, through a private contract?
Or would you rather eliminate the right entirely, so that if you fall in love with a foreigner, you have to hope they can make it to you through other immigration channels?
US Citizens who wish to petition USCIS to grant permanent residency to their fiancee or spouse must, as part of the petition process, sign a form that creates a legal contract with the US Government that the US Citizen sponsor must repay any means-tested social benefits the beneficiary receives. This is in addition to the filing fees incurred as part of the process.
Given that this bond must be created as part of the process, I could see your proposal as potentially being OK, except that USCIS would have to set up rules to limit this ability if the sponsor fails in their implicit duties to get the beneficiary on their feet. Basically, I see it as potentially being a privilege, not a right.