Ronald Moglia still remembers vividly the day he decided to study what makes children develop and think sexually.

"I was teaching third grade at an inner-city elementary school in the late '60s. One day after recess I entered the classroom, and all the children were lined up by the window.

"I told the kids to sit down so we could start, but no one moved. I had a pretty good rapport with them, so I was surprised. I asked them what they were looking at, and Corinthia -- the most vocal student -- said 'Mr. Moglia, they're doing it over there.'

"I looked out the window, and in a brownstone next door you could see a couple caressing and gyrating. I asked Corinthia, 'What do you mean by doing it? She put her hand on her hip and said, in a voice rather disgusted at me for being so dense, 'You know, DOING IT" -- and made a motion with both hands to graphically illustrate what she meant.

"I thought to myself, when I was in third grade I didn't know that. And I wondered how a third grader understands doing it .

"I said, 'Let's sit down and talk about it.' And from that point on I started teaching sex education to all my classes."

Moglia went on to earn a doctorate in science education -- specializing in the psychology of children's sexual development -- and is currently at New York University developing a sex-education curriculum for children in kindergarten through 12th grade.

In an age where half of all U.S. ten-agers engage in sexual intercourse (in metropolitan areas 70 percent of unmarried girls and 78 percent of unmarried boys have had intercourse by age 19), "We need different ways to look at and think about sex education," Moglia told teachers, ministers, health professionals and sec educators at a recent workshop sponsored by the Sex Education Coalition of Metropolitan Washington.

Moglia, 36, bases his work on seven factors "Most professionals in the feild believe":

1. "The process of sex education begins at birth and, by adolescence, many of te basic ideas are already formed. So if a school thinks they can start sex eductation then, they're missing the boat.

2. "Parental attitudes about sexuality are the strongest force affecting the child.

3. "The best source (for sex education) is the family -- but research shows parents are not doing the job.

4. "Most families favor some kind of family life/sex-education program.

5. "Other soical institutions -- peers, movies, media -- will fill the vacuum if kids lack the information at home.

6. "Scare tactics such as fear of pregnancy or veneral disease aren't an effective form of sex education.

7. "Sex-education programs document beneficial effects in children's psychological adjustment."

Counterbalancing these facts, Moglia says, "are these important realities: Despite growing recongnition of the far-reaching role sex plays in daily life, there exists a pervasive ignorance and misunderstanding about sex.

"No matter how enlightened some people are, the topic is emotionally laden for most adults. Communities are highly concerned about sex-education programs. And there is a wide variety of values in any community -- even in stereotypically homogeneous communities like Scarsdale or Watts.

"Many adults equate sexual knowledge with sexual behavior. They feel talking about intercourse is the same as 'teaching my kid how to fornicate.'"

In light of these facts and realitites, Moglia's proposed sex-education curriculum calls for a council comprised of informed adults -- parents, teachers, community leaders. They would choose from among a range of sexuality teaching materials those which they want taught in their schools.

"Their choices would be based on community values," he notes, "so a Mennonite community might have a whole different curriculum from a D.C. community.

"Let's say they decide sex roles, family relations, sexual behavior and reproductive biology are important. These factors would then be ingrated into the curruculum at all grade levels."

Rather than isolating discussion of sexuality into a separate unit, Moglia says, "It should be taught the same way we teach language -- all the time. When a fourth-grade social studies class is learning about pioneers, they discuss colloquial language of the period.

"That's the time to also talk about family structure and sex roles -- how, when the cowboy went on a round-up the women were at home, maybe for months at a time, and what that must have been like for each member of the family."

A basic problem of many existing sex-education programs, Moglia says, is that they either "try to teach about periods and wet dreams the day before each occurs," or they are "created by concerned adults who don't understand that kids think differently from adults.

"An example. When I was driving research at a nursey school, one 4-year-old forgot to pull up his pants after going to the bathroom. I thought, 'Wow, the other kids' behavior will give me my whole thesis.'

"Well, the class turned and saw him as they headed out to lunch -- and swooped by without a word. Except for one girl who said, 'Richard, you forgot to pull your pants up.' What adult is going to figure out that?"

To enable teachers to reach children at their level of understanding, Moglia bases his curriculum on the learning theories developed by the late Swiss biologist Jean Piaget and Lawrence Kohlberg of Harvard University.

"You've got to look at how kids think. If they are in an egocentric stage, they're not going to do well with abstract concepts of love. They understand love in terms of what it means to them -- 'I know my mommy loves me because she bakes me cookies.'"

But as important as a good school sex-education program is, Moglia stresses, "It's not a substitute for parental teaching at home . . . you've got to talk to your kids about sexuality from infancy on.

"The most important thing for parents to understand is that they have the right, and usually want, their kids to have their own values. But because many parents haven't been taught to feel comfortable talking about this area with their kids, their children upon adlescence find there's a whole area their parents haven't discussed -- and they wonder why.

"So they start to wonder if there are other areas the parents haven't discussed and question the validity of their values. If you're unsure of how or what to say, enroll in a community or church family-life and sex-education program for parents.

"Because not talking about sexuality is communicating something, too. And you can't be raised in this soiciety without getting sexual messages from other sources. So if parents want their kids to get their values -- not Jordache jeans' values -- they're got to communicate to them what their values are."