One of the few issues on which opinion has moved left over the last few years is same-sex marriage. In 1996, Gallup found that Americans opposed it by a 68 percent to 27 percent margin. Last May, Gallup found Americans in favor by 53 percent to 45 percent. That's a huge change in 15 years. Other polls have shown similar movement. Pew Research reported last week that 45 percent favored same-sex marriage and 46 percent were opposed -- a dead heat. Pew polls in 2008 and 2009 found only 35 percent to 40 percent in favor. This is an issue...