A marriage, certainly the licensing and recording aspect of it, falls under the "public records" verbiage of the FF&C Clause.
There is quite a bit of case law addressing how marriages that may be legal in one State will be handled in States where they are not. The full text of the FF&C assigns to Congress the task to figure out the manner in which those acts will be handled, but grants it no power to create wholesale exemptions.
Insofar as the slippery slope argument goes, that ship sailed the day that Protestants opted to walk away from marriage as an unbreakable sacrament.
While I agree that DOMA is likely to be overturned as to language, it won't necessarily follow the logic that was used to overturn Section 3 last year. And wholesale exemptions are what federalism is all about. As to public records, that would mean any license issued by a state must be recognized by all other states, and of course that's not true. So I don't think it's as clear cut as the proponents of same-sex marriage want it to be. It took until 1967 for the ban on issuance or recognition of interracial marriages in states to be overturned.
As for the slippery slope argument, like it or not, it's there. The public policy exemption, if it didn't exist, would basically toss out the 10th Amendment. And it's only when usage of the exemption violates either the Fifth or Fourteenth Amendments, does it come within the jurisdiction of the federal courts.
It's not out of the question that polygamy may one day be allowed in a liberal state. If so, I doubt most other states would accept that document should the group elect to move. Utah can't because it's unconstitutional in that state.