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. “I think people wanted to attach themselves to what may be the last great civil rights issue of our time,” Mr. Blankfein said.

Even the authors of the briefs seemed surprised by the wave of corporate support. “What’s remarkable is how fast this happened,” Joshua Rosenkranz, a partner at Orrick Herrington & Sutcliffe, told me as his brief was about to be filed on Thursday. “When I first started working on this issue, people said gay marriage was an idea whose time hadn’t yet come. Even people in the gay rights community said that. When we started to try to get corporate support, I was a little apprehensive. But I’ve been bowled over. Corporate America is not only already there, but they’re passionate about it. For them, it’s not just a human rights issue, it’s a business imperative.”

P. Sabin Willett, a partner at Bingham McCutchen and an author of the other brief, said his group, too, had to turn away companies. “I’d say this really metastasized during the last week,” he said. “Companies were calling and asking to join. We got a call from a Fortune 50 company on Monday. I said, ‘Can you get everything done in an hour?’ We could have had many more with another week.” The deadline for filing the briefs was Thursday.