But financial forces are at work against any changes. If the difference between a PG-13 and an R rating can be tens of millions of dollars at the box office, the last thing studios want is to slice the pie thinner. “In general, the more child-friendly the rating is  even for movies that might not be aimed at teenagers  the more tickets you sell,” said S. Abraham Ravid, a business professor at Rutgers University who has published many studies on movie economics.

Joan Graves, the chairwoman of the film industry’s Classification and Rating Administration, declined to comment on “It’s Complicated,” citing internal policy barring the public discussion of a specific picture. But she dismissed criticism of her board members.

“They react the way that most people react,” she said. “America is not just two coasts.”

Some in the industry see something deeper at work, arguing that the trade organization is on its best behavior because it has a lame-duck leader in Dan Glickman (who is to step down as chief executive in September) and because Congressional elections will take place next year. The Federal Trade Commission harshly criticized the movie industry this month for inappropriately advertising movies with PG-13 and R ratings to children.

It was not specifically the actual drug use that got “It’s Complicated,” about a divorced woman who has an affair with her remarried ex-husband, into this pickle, according to people with knowledge of how the decision was reached. Instead, the ratings board was concerned about what the movie did not have: a negative consequence for the behavior. (Ms. Graves said that “no scrutiny or outside influence impacts the rating of any film  period.”)

The board, according to these people, thought the scene was uproariously funny and could leave children with a strong message that smoking marijuana is fun. The opposite, of course, could be argued: One way to make young people think that marijuana is uncool is to show the white-haired Mr. Martin, 64, smoking it.

This emphasis on consequences has long been part of how Hollywood has navigated taboo subjects, dating back to the Hays Code era, said Robert Sklar, an emeritus professor of cinema studies at New York University and the author of “Movie-Made America.” “If somebody transgressed  infidelity, alcoholism  they had to pay for it,” he said.

The M.P.A.A. is often accused by conservative groups of “ratings creep,” a loosening of standards as the years go on, and of pandering to the studios, which resist R ratings because it could limit the audience. But “It’s Complicated” may be an example of the reverse.