How do you do that? Our laws governing pensions, survivor benefits, etc. depend on a definition of marriage and recording it. 20% of the population are atheists.
We have religions like Islam that permit up to four wives. How should they be treated?
The government has been involved with marriage since the inception of the Republic. We have laws against incest, polygamy, marrying children, etc. Should those laws be eliminated and leave it up to individuals to decide?
1st Amendment is Federal. It has a clause limiting it's scope to the FedGov under "Congress shall make no law"...
DOMA opened up a Federal can of worms and these are the fish you are catching.