It has always been the states that issued marriage licenses and the federal government has never done that. Each state has always made their own conditions for the basis of a marriage, what age you can get married. It has always been state law since the beginning of our country. It would be far easier to duke it out in your own state with your own state legislators than to try to do it nationally.
Something like he is proposing might also hold back a federal judge in making a ruling.
Wouldn't the federal law be required to cover D.C.?