WASHINGTON

In a San Francisco courtroom two weeks ago, a prominent lawyer opposed to same-sex marriage made a concession that could mark a turning point in the legal wars over the purpose and meaning of marriage.

The lawyer, Charles J. Cooper, has studied the matter deeply, and his erudite briefs are steeped in history. He cannot have been blindsided by the question Judge Vaughn R. Walker asked him: What would be the harm of permitting gay men and lesbians to marry?

“Your honor, my answer is: I don’t know,” Mr. Cooper said. “I don’t know.”

A couple of hours later, Judge Walker denied Mr. Cooper’s motion to dismiss a lawsuit seeking to establish a constitutional right to same-sex marriage. The concession and the ruling that followed it have transformed a federal lawsuit that had been viewed with suspicion by many gay rights advocates into something with the scent of promise.

The suit, filed in May by Theodore B. Olson and David Boies, made the bold claim that California’s voters violated the federal Constitution last year when they overrode a decision of the state’s Supreme Court allowing same-sex marriages.