There is a minimum age.
There must be sufficient mental capacity.
One and/or the other partner can not already be married to someone else.
One can not marry a non-human animal or plant.
There can be no third party in addition to the man and woman.
The list can go on. But the point is marriage was established by God. He makes the rules. Anyone who changes the rules is making a mockery of His institution.
You dance around the main argument.
There should be no civil marriages. The laws pertaining to civil marriage were enacted to prevent miscengenation - marriage between races.
Same with gun control laws. Those were enacted to keep guns away from “those people”.
Racism resulted in some of stupidest laws on the books. It is time we repealed them - and that includes putting one’s race down on census records. Enough!