SOMETHING WAS obviously amiss. The school year had begun and the choice was between sex ed and phys ed and the 13-year old announced he had selected gym.

"Why?" I asked, amazed.

"Because it's boring."

"Sex ed is boring?"

"Yeah. All they do is show the same movie we've seen every year and you don't learn anything new. Nobody's taking it this year."

So much for the Fairfax County sex education courses. It turns out, of course, that Fairfax County's sex education is notorius. Kids learn that they're going to grow hair and their voices are going to change, but they don't learn about contraception, masturbation, abortion and homosexuality. They don't learn about saying yes and saying no or how to decide. But if the Fairfax schools are notorious around here, they are apparently fairly typical of the rest of the country.

"The general public sincerely believes sex education is rampant in all the schools," says Planned Parenthood's Lee Bullitt. "Basically, nothing is happening. This is one of the myths that's around today that's perpetrated by the foes of sex education. They're pretending a lot is going on and that it really doesn't work and we're just teaching kids how to be promiscuous, when actually there is very little going on."

That kids are promiscuous and that they don't know much about what they're doing was dramatically illustrated Wednesday night on Harry Reasoner's CBS special "Boys and Girls Together." Reasoner cited the statistics: 2,000 teen-agers get pregnant every night, one out of five teen-agers has sex by the age of 13 or 14. He put the statistics together with some sleazy scenes of teen-agers cruising on a Saturday night, teen-age girls looking for a good time at the beach, and a truly decadent scene of a mob of teen-agers in drag waiting outside a hit transvestite movie. A girl wailed that you shouldn't judge teen-agers by them but by the time you got to that scene you couldn't be too sure.

Although it took CBS a year to find air time for the documentary, Reasoner gave an unprecedented public forum to the epidemic in teen-age pregnancy and casual teen-age sex sweeping the country and left it there for us to ponder. No upbeat ending."We couldn't think of any."

There isn't yet, but that doesn't mean there can't be. Dr. Peter Scales, who is currently studying barriers to sex education in 25 communities, says his research firm MATHTECH, estimates that "no more than 10 percent of the nation's schoolchildren can be said to have anything approaching a comprehensive sex education course." The course may touch on reproduction and veneral disease, but "there skills, the various kinds of contraceptives, the whole issue of saying no, how to behave in potentiallyy sexual situations, and the larger issue of self-esteem. How someone feels about themselves affects what they do sexually. These are arguably far more important issues to cover than when a woman's fertile period is, in terms of avoiding pregnancy."

Scales says most of the research of the effectiveness of sex education courses has been poor, but that certain trends have been consistent. The students who take sex educaton become more knowledgeable about sex, the programs don't seem to make them more permissive, and there is weak evidence that when contraception is included in accourse "they become more effective contraceptors."

Scales says school administration don't promote far-reaching sex ed courses because they are afraid of community reaction. But they may be misreading it or hearing only a loud minority.

"A couple a indicators tell us that parents in an overwhelming majority are very much approving of teaching sex ed, including contraception," Scales said. "Where parents are required to give permission, only one to three percent ever refuse. A recent study done in Cleveland on 1,400 parents found that the things parents want the most help on in communicating with children were just what the schools avoid: abortion, homosexuality, contraception, masturbation, emotions and values . . . when the parents are involved, given a chance to meet the teacher, see the materials, a lot of communities have found the parents asking for sex ed themselves because no one ever gave them any sex ed."

Michael Schaffer, a health education specialist with the Prince George's County Schools, says a comprehensive course is offered from the 9th grade up in which teachers can discuss "just about any aspect of human sexuality, (except) erotic techniques . . . but even enough we can do all of this, last year we only reached 6 percent of the students in grades nine through 12, because it's an elective course. It's not that they don't want it, but that they don't have time for it in their schedule.

"I see articles in the paper and I hear it on television people saying all that sex ed out there is causing the adolescent pregnancies. I say what sex ed? We're reaching 6 percent of the kids."

Scaffer would like to see sex ed mandatory with the consent of the parents. He points out that of the 6 percent who signed up for the course last year, 99.6 percent received approval from their parents to take it.

Bullitt says parents don't want the responsibility of teaching sex to their children and Schaffer's statistics suggest that she's right. For all of our talk, how many of us have actually sat down and given a course in sex education to our children? How many of us know enough?

As Reasoner says, there's always been teen-age sex, only there's more of it now and the kids are younger. Adults aren't going to prevent adolescent sex, but we can inform the children enough so they can choose whether to wait and so they can be careful.

We can give the kids a break a lot of us never had.