This may be a bad analogy but doesn`t it kind of boil down to what is the value of a dollar bill?
It is a meaningless piece of paper unless universally recognized as a tender instrument (a problem the Articles of Confederation had).
At some point marriage was assumed to be a Federal declaration whether legitimately so or not and so entwined into the tax code it will never be removed as such.
However I don`t see any reason any litigant cannot sue to apply the same standard,States rights be damned,for their own pet issue.
The courts at that point will simply close their eyes and declare what they think is the most comfortable and acceptable solution regardless of actual logic.
The law of status is an unusual battlefield these days, in our modern egalitarian society little remains of it, but once it was critically important in deciding exactly what laws governed an individual - Slave? Commoner? Nobility? Peer of the Realm? Clergy? Married, single, widow/widower? Birth order? Different sets of rights and obligations applied to each. There is not much of it left now, but some of the surviving categories that still matter are marriage, minority, and military service.
The real Federal question for the court was correctly identified by some of the DUmmies, which is the issue of whether a state must give the 'Full faith and credit' required by the Constitution to marriages performed in another state, even though such marriages are prohibited as morally repugnant in the state at issue. There is a very long record of the Supreme Court leaving the entire field of domestic relations to the states, but this case is not really about gay marriage as such, it is about what lawyers call 'conflict of laws' issues and the limits, if any, of the the full faith and credit clause.
Currency doesn't raise the same issue, since it is issued by the Federal government, not the states. I believe the Feds appropriated that role entirely to themselves long ago, but it's never been an area of interest to me, so I can't point to exactly where or when without doing a bunch of research on something I don't actually care about.
The Supreme Court generally tries to avoid deciding cases on 'What if' arguments, sometimes limiting the scope of their decisions if necessary to avoid the glaringly obvious horrible consequences if a particular decision if broadly applied could be used to bootstrap a whole lot of things that go way beyond the scope of the actual case they're deciding. But, there are some self-evident what-if scenarios that could arise from this, including how polyamorous or even adult incestuous (Not as impossible as you might think, there was some serious move in Austria to legalize them last year, as I recall) if any one state goes moonbat enough to legalize them.
