They aren't refusing to serve gay people, they can't do that. But most states in the US allowed people to be fired at will, for no reason, so if you don't like gay people you can fire the person if you suspect or find out that they are gay. They don't have any restaurants in California and I would bet that at least in some areas job discrimination on basis of sexual preference is illegal.
Actually they opened one (if not more) location in Los Angeles last year. Some people organized a protest out front, but nothing came of it. Apparently Neil Patrick Harris tweeted excitedly that a franchise opened there and people went ballistic.