Government in marriage is more than just adults living together. Marriage law, and by conection divorce law, has always been about children and providing for inheritance and support for the family.
The private system of marriage is completly inadequate to deal with property and inheritance rights. Governemnt came IN to recording marriages because the private system is too suseptible to fraud. Look at california's paternity fraud problem and multiply that a thousand fold with no minimum recording.
No one here is arguing against the point that one of goverments functions is enforcing contracts on childrens rights, property rights or inheritance.
I see no need for a Constitutional Amendment in those areas. Why do you?