Gay rights has been a defining progressive issue of Barack Obama's presidency: Congress repealed Don't Ask, Don't Tell in 2010 with his strong support, and in 2011, he instructed his Justice Department to stop defending the Defense of Marriage Act, helping to pave the way for a series of Supreme Court decisions that ultimately legalized same-sex marriage nationwide. This rapid succession of victories is astonishing by any measure--and is especially incredible considering that when Obama first took office he, like many politicians, still viewed gay rights as politically toxic. In 2008, for...