The 9th and 10th amendments and the U.S. Constitution at large has to demonstrate a clear and present danger, watered down to a compelling state interest for exercising any power over not specifically addressed in the Constitution.
Originally, the compelling state interest was the welfare of children, but after gay adoption was normalized, the only way remaining to maintain the Rule of Law has been for the States to simply get out of the marriage business.
People can contract together for any purpose, guided by contract law, under free association. It would also take heat off the Churches, allowing them to witness and perform ceremonies without the State having any interest aside from contract courts.
Prenups would be all the state had any remaining business becoming involved in.
It might teach the liberal scolds the difference between government and "society." A lot of people need to learn the difference.
I also suspect it was only a matter of time before government licensing of marriages became pointless anyway. Any God-fearing couple who takes their marriage vows seriously has no compelling reason to get a stupid “license” from the state.