[#3Fan] Congress can prescribe the way state acts are proven.
Proving a State law pursuant to Article 4, Sec 1 involves putting a State Seal on the document. Mission accomplished. See 28 USC 1738. (The Acts of the legislature of any State, Territory, or Possession of the United States, or copies thereof, shall be authenticated by affixing the seal of such State, Territory or Possession thereto.)
The Federal Government cannot interpret State laws. Should the Supreme Court of Massachusetts hold that the Constitution of Massachusetts guarantees the right of gay marriage, the Federal Congress and the U.S. Supreme Court are without authority to say squat about it.
The Federal courts review State laws to determine whether they violate the Constitution or applicable Federal law.
The Federal courts review State actions to determine whether the State acted in accordance with State law, i.e., whether due process was provided.
Article IV says the Congress may prescribe laws for states prove their acts and the effect thereof. It said nothing about it being one shot. It's not a marriage, it's an act with different effects on the other states and therefore Congress must have that opportunity.