When the Human Rights Campaign approached Lloyd Blankfein, Goldman Sachs’s chairman and chief executive, a few years ago about making a short commercial in support of same-sex marriage, Mr. Blankfein said he had the impression he’d be one of a number of prominent business executives taking a public stand. As it turned out, he was the only one. “It was a little lonely out there,” he said. It’s not lonely anymore. This week, Goldman Sachs was one of more than 100 corporations that lodged their support for same-sex marriage in two briefs filed with the Supreme Court.