...Yet no matter what the Justices decide after withdrawing behind their velvet curtain, the courtroom debate — and the period leading up to it — made clear that we have all been eyewitnesses to history. In recent days, weeks and months, the verdict on same-sex marriage has been rendered by rapidly shifting public opinion and by the spectacle of swing-vote politicians scrambling to keep up with it. With stunning speed, a concept dismissed even by most gay-rights leaders just 20 years ago is now embraced by half or more of all Americans, with support among young voters running as high...