Can you elaborate, please?

There's a concept of privity in contract law. Part of that has gone by the wayside, but the part that has always remained is that you can only bind yourself with a contract. If I sign a lease with a landlord and then want to add a felon to my lease, the landlord doesn't have to let the felon be added. If I sign a contract with this felon saying I promise to always add you to my leases, that's between me and the felon. The landlord isn't bound by my contract with said felon.

If I marry this felon, the landlord may legally (and morally) have to add him to the lease. Mere contracts don't have that power, but marriages do.

If you're angling at the idea that contracts should be restricted by the government, then there are some arguments for that

Yes, governments should refuse to enforce certain contracts. Glaring examples would be that if I contract with you to never marry a woman or to never sleep with an Indian or whatever, the government will absolutely refuse to enforce those terms.

Otherwise, you encounter the problem of those who have not chosen contracts, and society does not like that ambiguity.

How so? It'd simply refuse to aknowledge any marriage that is not written down as a contract. Common-law marriages are great, but surely we won't have common-law polygamous marriages. That'd be insane.

Congress has the power to make and unmake laws.

Sure, but if you can't rely on the government to have relatively stable marriage laws (at least where they count) then you can't do important marital things like division of labor. I can't quit my career if I don't know I'll be provided for even in a divorce. (or the flipside of child care). Etc.