In the decades since Roe v. Wade, public sentiment about abortion has remained fairly steady. By contrast, in the mid-1990s, only around a quarter of the country supported gay marriage, and then, somehow, just 15 years later, those numbers had nearly doubled. Sasha Issenberg, author of “The Engagement: America’s Quarter-Century Struggle Over Same-Sex Marriage,” tracks the twists and turns that the fight for same-sex marriage in America took, from a power struggle over a parade in Hawaii, to shifts in elite opinion, which all brought gay marriage from a “quirky,” niche issue in the 90s to being federally accepted by 2015.