As my example on state gun control laws go, there is no precedent for states having to follow other states regulations and laws. So what if SC must recognize a MA SS marriage license? But if SC law doesn't give benefits to SS marriages and only to opposite sex ones then it shouldnt matter. Can I bring Maryland traffic laws with me to VA. Can I use a radar detector there?
Correct so far, but it's early in the game. A court ruling in Mass. says that same sex couples must get federal benefits. This ruling could be overturned, but they keep chipping away, and they never stop. So if this ruling is upheld, it would almost certainly spread to other states.
You saw what happened with Roe v. Wade and the "right" of illegal aliens to get free education. Practices which were thought of as unacceptable are now part of the "American way of life," at least to the courts. Even when California passes a state constitutional amendment against gay marrige, they don't give up. If SS couples get federal benefits, that gives them momentum.
How does Holder get away with not defending DOMA? He wouldn't if enough people and politicians rose up against him.
Now that DADT is gone, they are delighted at the opportunity of making TV commercials showing gay "war widows" with no benefits. In Post-DADT Era, The Gay War Widow Waits
When USCIS denied a green card to a same sex couple, in a famous case, Napolitano granted a "humanitarian parole," turning a legal "sow's ear" into a political silk purse, at least from the left's point of view. They use executive power when they can, congress when they can, and the courts when they can.