Josh Barro's guess at its cause:
Only a handful of states have legalized gay marriage, but among upscale people in New York and Washington, opposition to gay marriage is now impolite. And expressing opposition in such a setting is exhausting. I’m not talking about "the Georgetown cocktail party circuit." I’m talking about Republicans politicians’ own wives and children, their young staffers, and even in a lot of cases, their donors. How many Republican members of Congress have children like Meghan McCain, who are reproaching them at home when they go out and talk about how terrible gay marriage is? I bet it’s a lot.