[Sir Francis Dashwood]

Congress may not however authorize the PERFORMANCE of any marriages that are not under federal jurisdiction, because that goes beyond giving effect to a document to actually usurping authority of the states.

No, the Constitution says... "the Congress may by general Laws prescribe ... the Effect thereof."

If Congress were to pass a statute declaring such said contracts null and void, they would be null and void for all intents and purposes of Law...

[old republic]

That power would not make contracts null and void. Congress only has power to to make regulations prescribing what EFFECT shall be given to documents. In other words, what the documents can be used for. The government can nullify the effect of gay marriage licenses in one state through this power, by making it of no effect in another state, but it cannot nullify the validity of that license within that state in which it was issued.