So what will be the next states to legalize gay marriage? Vermont? Rhode Island? Washington?
Rhode Island, New Mexico or New York are IMO the most likely. All three recognize out-of-state marriages but have no civil union provisions of their own. Hawaii (where the legal debate started in 1996) is also possible, as they grant some rights but not equal rights.
Other than those, states with statutes similar to the one in California which could have a court ruling overturn them include Illinois, Florida, Maryland, Delaware, Minnesota, Iowa and Arizona. Plus some states where I think such a ruling would be highly unlikely (or else would be likely to be overturned by an amendment to the state constitution).
