I think the main decision that is the true key to deciding who gets to define marriage depends on one crucial factor:
Is marriage a sacrament or a contract?
If it is a sacrament, then governmental involvement at any level violates the separation of church and state (in the US). Different treatment under the law depending on whether or not people are married is unreasonable, which would require many changes in tax codes, state and federal laws, Social Security guidelines, etc.
If it is a contract, then decisions regarding who can marry are moot—anyone who can legally enter into a contract can marry. There’s no reason to separate marriage contracts from established contract law.
Someone point out the holes in my argument, please? It seems all too simple to me.