Once upon a time, when I was a young liberal and my daughters were babies, I scorned the kind of narrow- minded, punitive Puritanism that led the Reagan administration to decree its so-called "squeal rule"--the one requiring parental notification when teen-agers seek contraceptives from government-supported agencies.

That was yesterday. Today I am the aging father of adolescent daughters and I understand the Reaganites' attitudes better. It is not, as I once would have thought, that they want to resurrect Victorian values by using pregnancy as the punishment for sex.

Rather, the squeal rule arises out of an impulse to strengthen the tattered fabric of family life in the intimate matter of sexual mores. Parental credibility in counseling "don't" is being ripped to shreds by television, movies, magazines and records that blare out the consistent message: "Do it!" "Everybody does it." "Even nice girls do it."

With so much salesmanship behind it (not to mention peer pressure and natural urges), the message is getting through. About half of all girls under 17 and 56 percent of boys are sexually active. Those percentages grow larger every year while the average age of first sexual involvement gets lower. It's now 15.7 years for boys, 16.2 for girls.

Any responsible parent has to view those trends with alarm. Sex is a grownup activity involving the most serious of life's consequences--intense emotional commitment, the possibility of birth and the possibility of death. The child who has taken up sex inevitably has moved away from his or her parents. Every year, parents are losing their children at a younger and younger age.

Along with the Reaganites, I abhor the Playboy magazine logic accepted by many liberals: face it, Dad, kids are into sex and the best thing to do is teach them how to do it responsibly.

Sorry, the responsible things for a 15- or 16-year-old to be doing are running track and learning math. Somehow, society--and certainly, the government-- ought to help parents to help their kids avoid sex before its time.

For all these reasons, I sympathize with the motivations behind the "squeal rule," but I still oppose it. It simply won't accomplish its intended result. It will just get girls pregnant. Right now, the rule is under court injunction, barred from taking effect nationally. An appeals court is to rule on it sometime this spring and probably the loser (the Reagan administration or Planned Parenthood) will take the case to the Supreme Court. The administration would do better by withdrawing the rule and trying something different.

The Reaganites obviously hope that when a letter arrives advising parents that their daughter has applied for contraceptives, a reasoned, compassionate family discussion will ensue, leading to a decision that daughter should wait.

The problem is that, in the vast majority of cases, the issue of waiting or not waiting is long over by the time girls go to the family planning clinic. Surveys taken at family planning clinics show that only 14 percent of girls seek contraceptives before having sex for the first time. On the average, the clients have been sexually active for nearly a year, often without benefit of birth control.

I am not bothered at all by the alleged sex discrimination implicit in the squeal rule--the fact that it affects girls and not boys (who can get their contraceptives at the drug store, no questions asked). If the Reagan rule worked, parental notification would prevent pregnancies, and it is girls who get pregnant, not boys.

But the rule will not work and it does discriminate against poor girls who cannot afford to see a private gynecologist and get a confidential prescription.

Poor families are less inclined than better-educated ones to have the reasoned, compassionate discussion the Reaganites hope will follow parental notification. Poor kids tend to get involved in sex earlier than wealthier ones, to be less informed about birth control and to produce more babies out of wedlock. The Reaganites want to cut back on AFDC, but the squeal rule only swells the number of potential recipients.

Poor or rich, the girl who thinks she can tell her parents she is going for birth control will tell them, and parental notification won't be needed. The girl who can't tell her parents--who fears their knowing--may well be discouraged from going to a clinic.

The administration claims the squeal rule will cut clinic caseloads by only 4 percent. That seems a ridiculous underestimate. Planned Parenthood asserts the correct figure is 25 percent. Either way, though, it's certain that the rule will increase the number of teen-age pregnancies, abortions and unwanted babies. There already are too many of all three.

Parents do need the government's help in influencing the sexual attitudes and behavior of their children, but the time for exerting influence is long before they arrive at the door of the birth-control clinic.

The Reagan administration should withdraw the squeal rule and devise programs to educate parents on how to talk to their children about sex and values. And it might help, too, if President Reagan would speak to his friends in Hollywood about the extent to which they have oversexed American society.

The writer is executive editor of The New Republic.