During the span of 25 years, same-sex marriage went from being an unimaginable idea to settled law. The data behind that evolution is striking. At the beginning of the millennium, about two-thirds of Americans opposed same-sex marriage, and a third supported it. Today those numbers have flipped. We speak with journalist Sasha Issenberg about how that happened. His new book is called "The Engagement: America’s Quarter-Century Struggle Over Same-Sex Marriage."