[Wolfie]

What does the Constitution say about it?

[Edward.Fish]

What does the Constitution say about it?

Amendment IX
The enumeration in the Constitution, of certain rights, shall not be construed to deny or disparage others retained by the people.

Amendment X
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.

So, legally speaking, I think that he's right.
Marriage being outside the purview of the federal government is obvious, as there is no mention of it within the Constitution — though it could be argued that it's not a matter of either States or the People either, but instituted by God… in all these cases it's not within the federal government's authority.

Not that the self-styled god-kings on the judiciary would ever use such simple and encompassing reasoning.