The mainstreaming of homosexuality took a long time. Gore Vidal’s The City and the Pillar, a literary mediocrity but a cultural milestone, was published in 1948. Fifty-five years later, homosexual acts remained criminal in much of the United States, until Lawrence v. Texas was decided. The culture wasn’t quite sure where it stood: Americans are generally liberal about these things, and positive depictions of gay characters had been an ordinary part of American culture for decades. But even after Lawrence, both Hillary Rodham Clinton and Barack Obama felt it prudent to run as candidates opposed to gay marriage in the …