

I think you missed the basic point: if it doesn’t apply to all citizens it cannot apply at all. That is not loser talk. That is simple to the point and is exactly what you want in a government.
I appreciate everything else you said however. Relating it to the real world situation is reality.
A conservative would ask you why the woman needs to be an adult?
Again, the only real answer is you have an adult citizen who wants to form a contract (because marriages are a social and/or religious concept) with another citizen. Done.
Exactly what you said: it is a legal contract with downstream consequences (well not really if you dont want to bother with it at all, but I digress). A legal contract. You cannot pick and choose who gets to make contracts, because if you do, they really have no legal basis.
Red heads cant sign contracts with left handed people. That is where you will end up.