Driving along the L.A. freeway, his radio tuned to a top-40 station, Norman Fleischman got a message over the air. Paul Anka, the '50s guru of clean-cut teenism, was crooning his heart out. . . . "You're having my baby. What a wonderful way to say you love me."

Fleischman nearly wrecked his car.

"With a million teen-age pregnancies a year, I heard that and said there's just got to be a way in this industry to do something . . . rather than promote the idea of teen-agers having babies."

Fleishman, 48, father of four, did two things: jokingly offered Anka -- father of five girls -- an honorary vasectomy, and started the Rock Project.

That was six years ago. Today the Rock Project continues to use stars like Linda Ronstadt, Loretta Lynn, War and the Commodores to produce radio spots promoting sexual responsibility among teen-agers.

Along the way the project has spawned the Sports Project with athletes like Reggie Jackson and Julius (Dr. J) Erving appearing on consciousness-raising posters. A hotline was opened early this year in the Washington area.

Around 500 radio stations, says Fleischman, have played the spots nationally, including Washington stations WPGC, WOL and WMZQ. City and suburban teachers and youth leaders have peppered recreation centers and classrooms with posters espousing the project's philosophy: Nobody wins with an unwanted pregnancy.

"A child is not like a piece of clothing," begins one poster endorsed by the Commodores. "If the style changes, you can't get rid of it."

"Cardozo High School has a high number of pregnancies," says Michael Gallion, explaining why he uses the posters in physical-education classes. "We make the information available. At least they know they're not alone."

"What is needed is information," adds Vance Wisdom, director of Fairfax County's Roving Leader recreation program for pre-deliquent youth. "We need all the help we can get."

His father, he recalls, told him as a youth in New York City "to wear my rubbers in the rain." Such veiled euphemisms were all right then, he says, but don't work today. "Teen-agers want hard, cold facts."

The posters and radio tapes are produced by the Center for Population Options, a non-profit group that originated partly from that Anka serenade. Funding for the center, which focuses on adolescent pregnancy, is provided largely by private donors.Fleischman also serves as a media consultant to TV writers and producers concerned with portraying teen-age sexual relationships more realistically.

The tapes are sensitive, sometimes witty, off-the-cuff comments taken from interviews conducted by the center.

"To have a kid in order to leave home is out of the frying pan and into the fire," says rock star Janis Ian.

Country singer Charlie Daniels: "It takes a lot of time to raise a child. It's not a damn automobile, something you can take home, and try it, and if you don't like it, take it back."

Commitment, maturity, friendship are a few of the themes center workers stress in their attempt to encourage mutual respect among teen-agers. Parenthood, they emphasize, should be a choice, not a consequence.

Among facts listed by Carol Fontein, the center's East Coast representative:

About 3,000 teen-agers become pregnant every day in the U.S.

The District of Columbia leads the country in teen-age pregnancies.

A 1980 teen environment study, sponsored by the Junior Achievement national board of directors, indicated that peers, parents and the electronic media, in that order, are the greatest influences in a teen-ager's life. Eleven years ago it was parents, teachers and peers; the media was eighth.

Sexual myths abound. Among those Fontein has heard: A Coca-Cola douche prevents pregnancy; Saran Wrap can be used as a condom; everyone gets pregnant the first time they have sex.

It is the center's hope, says Fontein, that the tapes and answer line will provide information, "correlate some of the myths" and nudge youths into thinking about the risks, as well as the pleasures, of sex.

The media, she says, "teases" them. "Sex is used so much in advertising.

"Here are these kids, half children, half adults, being encouraged every which way to have sex, but not being given the information they need to act as responsible adults."