Gov. Faubus of Arkansas tried that when a federal judge ordered Little Rock Central High School desegregated. President Eisenhower federalized the Arkansas National Guard, and called out the regular U.S. Army just to make sure the Guard obeyed the federal officers. That was the last time any state tried to defy a federal court order.
And no, I am not comparing desegregation to gay marriage. I am pointing out that federal court orders can be enforced by the President with a lot more firepower than the states can call upon.
Make it clear that an attempt to enforce the order means civil war.
Do you think the Army would fire on Americans to enforce gay marriage?