In matters of adolescent sex, the more things change, the more they stay the same. Except for references to the latest sexually provocative television commercials and a growing teen-age acceptance of sex between steady partners, a recent discussion by young people at a Planned Parenthood conference in Montgomery County seemed a throwback to the years before the baby boom.

"Sex has been cheapened," said panelist Mary Bodnar, 17, a senior at Albert Einstein Senior High School who said she wants to be married before she enters into a sexual relationship. "In TV shows and what you hear on the street. It's been made ugly."

"After the first time, most guys feel . . . victory. Girls feel shame," said David Lucas, 16, a Sherwood High School junior who leads Planned Parenthood rap sessions on contraception for other teen-agers.

Planned Parenthood's Debra Haffner said last week at the workshop, held for educators, social workers and other service group representatives, that the favored form of birth control among teen-agers hasn't changed, either.

"Just like in 1940, when boys went to the drug store and bought condoms," she said, "today, boys go to the drug store and buy condoms."

The number of births to local teen-agers increased slightly last year, when 1,850 suburban Maryland females between the ages of 13 and 19 had babies, 20 more than in 1981, according to the Maryland Center for Health Statistics in Baltimore.

But while the number of pregnancies has increased, the reasons--reflected in teen-ager attitudes toward sex and birth control--seem to echo those of previous generations.

Six panelists last week talked, among other things, about how they learned about sex, why they believe people their age often talk about sex without mentioning contraception, and why--in the view of one unmarried high school senior--having a baby seems a reasonable course.

Lucas said teen-agers sometimes bring shocking misinformation to his rap sessions.

"They tell me they've heard you can't get pregnant if you do it standing up," he said. Instead of showing films in biology classes explaining the reproductive system, Lucas advocated the organization of small groups of students where birth control devices can be passed around and explained. He said teen-agers need to advance their sexual educations in settings where they feel free to ask questions.

Sonja Williams, 19, a single mother of two from Gaithersburg, told the audience that her first significant introduction to sex education--when someone finally sat down with her and explained about birth control--came "when I was five-months pregnant" and staying temporarily in a home for unwed mothers in Northwest Washington.

Williams said she had wanted to get pregnant, although it was a surprise to the baby's father. She said that her upbringing led her to rebel: she was strictly forbidden to have boyfriends and could not ask the grandparents who raised her about sex. She later had a second child--again, intentionally, she said.

After the session, Williams said her reasons for wanting a baby included the fact that, until then, she "never could get anyone to take me seriously. I did it also to get my grandfather's approval."

Williams added that she agreed to participate on the panel because, "I want to help other people." Although she said she had "no regrets" about having had the children as an unwed teen-ager, "I don't want anybody else to go through it."

Throughout the discussion, both the teen-agers and representatives of the sponsoring groups stressed that communication about sex within families is crucial to help teen-agers cope with peer pressures. But, said James Farmer, executive director of the Community Action Agency, "It's obvious to me that a lot of parents are not talking to their children."