Marjorie Mecklenburg, the Reagan administration's chief enforcer for adolescent pregnancy prevention, went into the liberals' den last Friday and delivered an object lesson on how effectively conservatives have co-opted social issues.

Mecklenburg, a former leader in the antiabortion movement, drafted the administration's proposal requiring federally funded clinics to notify parents when minor children get prescription contraceptives. Despite a conciliatory attitude, she had no chance of winning over her audience--the National Family Planning and Reproductive Health Association--to the administration's position.

Mecklenburg told her audience that she is committed to involving parents in teen-age family planning. "Sexuality is one of the most difficult things for parents to get involved with," she said, and stressed that the department wants to develop ways of helping them. And, she said, "we would like to see more teen-agers not sexually active. I'm not ashamed to say that." If they insist on being active, "we want them contracepted safely and contracepted effectively." It was her only applause line.

But in the end, she hit a home run right out of the East Ballroom of the Washington Hilton by speaking to fundamental values that hold societies together while her questioner spoke to values that let them fall apart.

What, said a family planning provider, about parents who don't wish to know when their children seek contraception. "What," she asked, "about their desire not to be informed?"

The audience applauded.

Mecklenburg responded: "Parents cannot shift responsibility for children onto professionals or some other group in the community on something as basic as sexuality. . . We believe parents do have a responsibility to be part of the team."

The argument against notification is that some children, afraid of parental reaction, will not go to the clinics for contraception and will get pregnant. In fact that may be true, as at least one study has suggested. But by focusing on this aspect of the teen-age sex question, liberals are left in the position of going to bat for parents who want to ignore what their children are doing, and of defending a public policy that allows the federal government to provide, without notifying parents, contraceptive prescriptions to children who are considered underage by state statutory rape laws.

Mecklenburg struck a note far more in tune with American beliefs about parental obligations. Liberals, concerned with issues of privacy and leery of anything that appears to impose moral and religious values, have failed to develop a sensible, politically appealing alternative to solving the problem of teen-age pregnancy.

Dr. Sol Gordon, director of Syracuse University's Institute for Family Research and Education, receiving the American Society of Sex Educators, Counselors and Therapists' annual award in New York City last weekend, called the epidemic of teen-age venereal disease and pregnancy a "national social disaster."

Notification of the parents, however, is not the answer, he says. The answer lies in part in encouraging parents to be the prime sex educators of their children, in teaching teen-agers that premature sex is a health hazard, and in greatly expanded and improved sex education programs that address such questions as love and decision-making and that promote self-esteem.

"The women in college now are not available for exploitation," says Gordon. "We are seeing more and more egalitarian relationships. Our best hope is if the message of the women's movement gets down to teen-age girls."

There is, as Gordon has outlined, a responsible liberal program for reducing teen-age pregnancies. Notification requirements involve families but may also increase pregnancies by discouraging contraception use. There is a need in this debate for liberals to develop and articulate a program that puts prime responsibility for teen-age sexuality back on the parents, that will educate teen-agers about the hazards of premature sex, and that will give teen-age girls the self-esteem necessary to withstand the peer pressure of their boyfriends.

By failing to focus the debate on the need for sex education that has values and on the necessity for parents to assume their responsibilities toward their teen-agers, the liberals are simply dropping the ball and letting the conservatives run with it.