Ha, less than 1/5 of the states allow same sex couples to get married. That's hardly what I'd call "many." It's more like just a few.
And to put this into a different perspective, as a neighbour of yours to the north, Canada has been ahead of the US is virtually every civil rights issue, including slavery and women's rights. And the way this country separates the powers between province and country is a whole lot closer to how the U.S. government works compared to how the U.K. government works.
It's not the system of government that makes the difference. It's the attitudes of the people that does. And the U.S., in my experience (yes I've visited on several occasions), is not a very friendly place for people who are gay. I am more free here, as a Canadian, than I would be had I been born American.
The U.S.A. - Land of the free, unless you're gay.