"We constantly hear from our kids how impossible it is for some of their friends to even talk to their parents about sex. It is absolutely imperative that federally-funded family planning clinics maintain the kids' trust and confidentiality," write the "concerned parents" of two Weston, Conn., teen-agers.

But the chairman of the Cattaraugus County chapter of the New York state Moral Majority complains that "parental authority and responsibility have been undermined to a critical point in recent years" and voices whole-hearted support of a government proposal to do something about it.

Their letters are among thousands in bags and boxes of mail piled up on the seventh floor of the Department of Health and Human Services' Hubert Humphrey Building. Officials there estimate that a controversial proposal that would require federally funded family planning groups to notify parents when teen-age girls get contraceptives has drawn more than 30,000 comments.

Observers say that it is one of the largest public responses in the department's history. But until the overwhelmed family planning staff gets the mail opened and sorted, no one will know for sure how many letters have arrived. The official 60-day public comment period ends today.

Marjory Mecklenburg, the HHS official in charge, refused to comment on the general tone of the mail, saying that she and her staff had not had time to read it yet.

But an overwhelming majority of hundreds of personal letters sampled by the Washington Post were against the government's plan--as many as four out of five in some unsorted batches. It is impossible at this point, however, to predict the final tally.

The proposal would require federally funded family planning clinics to notify parents within 10 days after a young woman under 18 gets a prescription birth control product. HHS based its argument on health considerations, saying that parents have a right to know when their children are receiving potentially hazardous prescription drugs.

However, negative letters from parents, teen-agers, doctors and other professionals sounded a common note of concern that the result will be unwanted teen-age pregnancies that would result in even greater physical and psychological, as well as economic, harm.

Many were poignant, individual appeals to reconsider the proposal. "I might have ended up being a pregnant teen-ager," wrote one young woman who said she took advantage of the confidential services now available at a family planning clinic. "I know I would have been really scared to talk to my parents about it. They are old fashioned and sex was never mentioned in front of us kids."

Others from around the country complained that the proposal was discriminatory toward women and the poor, that it interfered with doctor-patient confidentiality, that it overrode state and local rights, that it contradicted the Reagan administration policy of getting the government off the individual's back and that it was impossible to legislate morality or better communication within the family.

A Madison, N.J., women called the proposal "an attempt to force 'Moral Majority' morality on Americans--the administration is using this to try to force teen-agers to stop being sexually active. It won't work. You cannot force teen-agers and their parents to communicate on the subject of sexuality."

One "lifelong conservative" doctor even threatened to withdraw further support to the Republican Party "until these disastrous rulings are withdrawn."

Proponents' letters, many from abortion foes, seemed to support the notification plan largely on moral and religious, as well as health, grounds as a "step in the right direction." A Boothwyn, Pa., woman predicted that "there will come a day when teens will thank parents and the government for guiding their lives toward a moral life."

Many felt that parental consent was warranted before contraceptives were given to teen-agers and urged the government to go further. One father of two, dating the decay in current morality back to Elvis Presley and his onstage pelvic gyrations, congratulated Mecklenburg on her "campaign for continence."

While many letters appeared to be spontaneous, some were part of organized campaigns by national and local groups on both sides of the issue.

Department sources said that it was unlikely the negative mail would sway the government to drop the notification proposal .