For Marjory Mecklenburg, the enemy is popular culture, tempting teen-agers with seductive images of casual sex.

"There is this environment that is very suggestive," said Mecklenburg, captain of the Reagan administration's campaign against adolescent promiscuity. "There are messages that are beamed to our young people to become sexually active, that this is glamorous, that it is modern."

The belief that it is the government's role to combat these messages is the raison d'etre of the Adolescent Family Life Program, a $13.5 million federal initiative to discourage teen-age sex.

Conceived two years ago by Sen. Jeremiah Denton (R-Ala.), the proposal was quickly derided as the "chastity bill" because one of its legislative directives was to "promote self-discipline and other prudent approaches to the problem of adolescent premarital sexual relations."

Can the government succeed where parents, teachers and clergymen have failed?

The jury is still out, but under the guidance of Mecklenburg, a Minnesota anti-abortion activist who now serves as deputy assistant secretary of Health and Human Services, the Adolescent Family Life program is playing a growing role in presidential policy to strengthen the American family.

That policy has been the center of controversy, much of it focused on Mecklenburg's proposed "squeal rule," another derisive nickname given to a directive requiring parents to be notified when teen-agers receive contraceptives at federally funded health clinics.

But while the squeal rule has been squelched temporarily by a federal judge in New York, the chastity program, its ideological twin, marches forward, albeit with heckling from family planners and other groups.

Last fall, Mecklenburg's office received more than 500 applications for money to do demonstration and research projects and gave funds to 62. In Reagan's new budget, it is one of the few public health programs slated for an increase in funding, a requested $16.3 million in fiscal year 1984.

"The need is tremendous," said Mecklenburg, a mother of four and wife of an obstetrician-gynecologist. "Most people realize that very young teen-agers who are sexually active run a risk of pregnancy and venereal disease."

Some of the awards, especially on the research side, appear to have gone to longtime federal grantees who simply rewrote old proposals to match the new focus of Adolescent Family Life. But others apparently have come up with different approaches to a perennial problem.

A public health group in Minnesota is using its grant to produce cable television programs promoting sexual abstinence and targeted at rural teen-agers. In Baton Rouge, a state agency proposes to select teen-agers to serve on an advisory board as "role models of responsible sexuality for adolescents."

There are to be "living room dialogues" in Massachusetts. In Arlington, Va., Catholic Charities is organizing workships that will train parents as "sex educators."

Critics say many of the grants confirm their initial reservations about the idea. The National Urban League recently fired off a letter to congressional oversight committees charging that the awards went to only one minority applicant and bypassed many inner-city areas where the problem of unwed mothers is particularly acute.

A spokesman for the American Civil Liberties Union in New York said the group is considering challenging some of the awards to religious groups, including the one to Catholic Charities in Arlington, on the grounds that they promote religion with federal funds.

"The legislation itself is worded in such a way that it encourages sectarian grants to be made," said Janet Benshoff, director of the ACLU's Reproductive Freedom project.

The ACLU's concerns are echoed by family planning groups who question the practicality of efforts to graft religious morality onto federal programs.

The 17-member Office of Adolescent Pregnancy Programs headed by Mecklenburg was created by the Carter administration as a service program for pregnant teen-agers. But Denton, later joined by Sen. Edward M. Kennedy (D-Mass.), put a new twist on things.

Beside the promotion of sexual abstinence, grantees were barred from mentioning abortion and directed to encourage pregnant girls to give birth and put their babies up for adoption. Tight restrictions were placed on providing information about contraceptives or birth control methods.

"The whole emphasis is on discouraging teen-agers from having sex and then forget about them until they come back in pregnant," said Asta Kenney of the Alan Guttmacher Institute, a research affiliate of Planned Parenthood.

But Nabers Cabaniss, staff director of Denton's Labor and Human Resources subcommittee, said the program's fundamental goal is to prevent such dilemmas.

"You can't say that teen-age pregnancy is a problem and that sexual activity has nothing to do with it," Cabaniss said. "There's a cause and effect there and you have to get at the cause."