Explain to me how a state constitution supersedes the federal constitution regarding same- marriage.?

I don't know for sure, but I'm guessing that when a state court declares a gay marriage ban unconstitutional, it is in terms of the STATE constitution. These decisions happen in state supreme courts, and only affect the state. Anything dealing with the federal constitution would be happening in federal courts, and if gay marriage bans were found unconstitutional here, it would affect the whole country.