If you look at the new statistics on sexual activity among teen-agers, you might wonder what else in their youthful lives they have time for besides mating. Last week, a study published in "Family Planning Perspectives," a magazine of the Alan Guttmacher Institute, reported a booming increase in premaritial sex among unmarried teen-aged women.

Only 10 years ago seems like the age of innocence; a "mere" 40 percent of those surveyed had had sex by age 19. In 1976, the figure was nearly 60 percent. And now it is nearly 70 percent.

Because it deals only in numbers, not attitudes and much less moral values, the study leaves us guessing about what kind of sex -- responsible sex, promiscuous sex, romantic, confused or peer pressure sex -- the young are so fulsomely engaged in. What isn't left to guesswork is the reality that in the sexual revolution of the past 20 years the ones most victimized by the upheaval have been the most defenseless, the young.

For most of us over 40, decisions about sexual activity didn't have to be made until we were in our early 20s. The timing was synchronized with other decisions: career choices, education and political leanings. Now it is different. Before today's young must ever think about earning a living or how to increase their intellectual independence or begin creating a workable philosophy of life, they are forced to deal with what was described on the dust jacket of the recent book, "Teenage Sexuality": "The importance of sex, masturbation, orgasm, virginity, homosexuality, fantasies, performance anxiety, response to pornography . . ."

Whatever personality breakdown a case of "performance anxiety" may lead to, the early 20s offer more emotional and economic security to handle it than the early teens. At 22 or so, the individual is on his way to becoming value-conscious, having passed through value-free years of early childhood and the value-neutral years of adolescence. To understand one's sexuality and a committment of love to another person is impossible without at least a small idea of what is genuinely valuable in life and what isn't.

As the new statistics suggest, the young have been duped into thinking that early sex is not only an easily mastered art, but a consumer item they should neither deny themselves nor be denied by moralizing old Puritans. That this sham is dumped onto them is partly due to the pressure exerted by some of the planned parenthood groups. Their main argument is that since sex is inevitable among adolescents, our major duty is to provide the technology for "safe sex." When the pregnancy rates continue to rise, an "epidemic" is declared, twinned with a call for bigger and better contraceptive programs.

The shallowness of this technological solution was exposed recently in Commonweal by Margaret Steinfels: "If youthful pregnancies or their possibility are a 'disease' (as in 'epidemic'), contraception or abortion are the 'cure.' The larger moral, social and political issues are then defused by shifting the 'disease' and the 'cure' to neutral territory -- the hospital and the doctor's office."

This doesn't work, because the one thing adolescents don't need is neutrality. They are struggling to move through the value-neutral years of adolescence to those ahead when they can be value-conscious and grounded. If teen-agers have trouble talking about their personal moral lives, it is not because they don't have them but because they feel overwhelmed by a society that does little to reinforce the enduring values.

So they feel isolated. Or scorned. The pressure is on -- from movies, records, advertising -- to accept the prevailing view of materialism, that all that really counts is the pursuit of money, good looks and easy pleasure. A fear exists that we shouldn't talk to adolescents about moral values. We might stunt them. Worse, we might sound like Jerry Falwell or the current pope as he rails against the dread sin of leering.

The risk is worth taking. Those who work in the federally funded comprehensive service centers for teen-age mothers report that adolescents are hungry to have adults level with them about value systems that have lasting meaning.

Contraceptives have a useful role, but it isn't only what comes out of a drugstore that is effective against the painful consequences of early sex or unwanted pregnancy. What comes of tradition, or non-sermonizing religion, or counseling services provided by groups that believe adolescents are something more than rutting animals -- these approaches would work also, if we are daring enough to try them.