Early in Norm Macdonald’s new film, Dirty Work, the comedian is thrown in jail after a scheme goes awry. No sooner does the comic voice a fear of what happens to soft guys in prison than he is taken away by three hulking bikers.

“Hey,” he says with the mock severity that was his trademark on Saturday Night Live. “You fellows have a lot of growing up to do.”

It’s a big laugh line, but it won’t be in the movie when the MGM film arrives June 12. Making fun of sodomy didn’t play well with the Motion Picture Association of America, responsible for assign ing ratings that suggest what films are OK for children.

“That was nonnegotiable _ if we’d kept it, we would’ve had a R rating,” said producer Robert Simonds. “You just can’t have the star of your movie reprimanding his tormentors for being [violated) and still get a PG-13.”

In today’s Hollywood, the important grade for youth comedy is PG-13, introduced in 1984 to provide a middle ground between the nearly-anything-goes R and the more restrictive PG rating, which severely limits a film’s language and sexual content.

Since 1984, R-rated youth comedies have become an endangered species. In fact, since the release of the R-rated Eddie Murphy: Raw in December 1987, 27 youth comedies have earned more than $45 million in domestic box-office business. All were rated either PG or PG-13.

“Movie studios are really following the PG-13 mandate,” says Gerry Rich, president of worldwide marketing at MGM Pictures. “For parents of young teens, it really provides a ‘Good Housekeeping seal of approval.’ Our production and marketing budgets are so much higher these days that a PG-13 rating is a logical choice _ it gives you a broad er range of distribution and more freedom with TV advertising.”

But has the economic incentive to earn a PG-13 rating been responsible for a fall-off in quality in present-day youth comedy? It’s hard to find comedies today that are as universally beloved as films like Animal House, Stripes and Caddyshack, which all were R-rated films.

“PG-13 has killed comedy in modern-day movies,” says writ er-director Roger Kumble, who is currently shooting Cruel Inven tions, a high-school sex comedy with Sarah Michele Gellar and Reese Witherspoon. “Once PG-13 got established, you couldn’t be outrageous and politically incorrect. Think about the legendary (caught-in-the-act) scene between Phoebe Cates and Judge Reinhold in Fast Times at Ridgemont High. You could never do that today and get a PG-13.”