Evan Wolfson, the president of Freedom to Marry, said he welcomed a conversation, but the arguments against same-sex marriage were so empty that they were not worthy of respect. “If you know that the only arguments that can be made for a position are discriminatory and harmful to real people,” he said, “you should think about whether you should make them.”

This latest skirmish in the culture war over marriage was prompted by the Obama administration’s decision in February that it would no longer defend in court the part of the Defense of Marriage Act that denies federal benefits to gay and lesbian couples married in states that recognize such unions. That decision was itself unusual and thus telling.

But it was only one indication of how quickly the battle lines are moving. In 2008, a federal judge in New York ruled that it was defamatory to call a straight man gay. Ten months later, a different judge of the same court, relying on what he called “a veritable sea change in social attitudes about homosexuality,” said there was no longer “a widespread view of gays and lesbians as contemptible and disgraceful.”

The second judge, Denny Chin, drew a comparison. In 1926, he said, New York’s highest court ruled that it was libelous to call a white man “colored” or “Negro.” Such rulings were common in much of the nation in the first half of the last century; they are unimaginable today in any state. The range of views that may be expressed in respectable circles can be a bellwether in judging what society is ready for, said David A. Bositis, an analyst at the Joint Center for Political and Economic Studies who has studied the politics of race.