I learned this in traffic school. In many cases, something that is state-governed, is individual to that state. Massacheusettes said you guys could marry, but Florida disagrees. Since Florida doesn't allow gay marriage, they're allowed to deny recognition of it. Some professions are the same. Different states have different guidelines and restrictions so you can get a business liscence in one state that isn't recognized in another because they have differing restrictions and guidelines and policies. Since marriage is a state-to-state affair, each individual state has the right to not accept something it doesn't support itself.