Two days after legalizing same-sex marriage in New York, Gov. Andrew Cuomo marched in the New York City gay pride parade. "You're going to see this message resonate all across the country now," he predicted. "If New York can do it, it's O.K. for every other place to do it."But most states can't do what New York did. Their legislatures can't legalize gay marriage, because their voters have passed ballot measures that prohibit it under their state constitutions. The ballot measures were enacted ye...