Despite the strong desire of mothers and fathers to provide for their child's sexual education, when it comes to discussing topics such as intercourse, premarital sex, venereal disease, pregnancy and contraception, most parents still retreat into silence.

These are the recent findings of the Project on Human Sexual Development, whose researchers spent nearly three years examining the attitudes of 1,400 Cleveland parents. The respondents represent a cross-section of middle America, ranging in age from 16 to 60, and encompassing a broad range of occupations, races, religions and education.

Other findings: Only 15 percent of mothers and fewer than 8 percent of fathers have ever discussed intercourse with their children; parents are more likely to approve premarital sex or masturbation for sons than for daughters, and many parents are unsure of the accuracy of their information and the utility of their values.

"discussing sexuality with a child should be as normal and natural a part of parenting as possible," says Elizabeth J. Roberts, executive director of the Cambridge-base project and mother of a 10-year-old daughter.

A former Harvard professor and coordinator for the White House Conference on Children, Roberts said her first reaction to the statistics on teenage sexuality was, "My God, we're surely not preparing our kids for this.

"the earlier you begin (discussing sexuality with your child), the better. Three, 4 or 5 years old is not too young for a child to know that their bodies are an okay subject to talk about."

Roberts says bathing a child is a good time to begin discussing parts of the body and the difference between sexes. She first told her daughter where babies came from during bath when the child was 4.

"i said babies came through men and women using their bodies in a loving way and continued the facts in a simple, straightforward manner.

"but it's not a one-time birds 'n' bees talk. These topics all need to be re-raised as children grow."

Stressing that "You don't have to know all the answers or be a sex-education expert to start a discussion." Roberts offers these suggestions for parents:

Don't be afraid to say, "I don't know, but I'll find out." Go to the library and get some books for yourself or to read together with your child.

Get together with other parents and organize a discussion group with a speaker from a local university or sex-education center.

Let your child see you and your spouse showing affection to each other. "A child learns about sex in a variety of ways, whether you talk about it or not. If parents stop hugging when the child walks into the room, the child learns that their parents are embarrassed or ashamed about what they are doing."

Get fathers involved. Boys and girls have a right to hear both parent's fellings about sexuality.

If you are nervous when broaching the subject, say so, since your child will probably pick up your apprehension. You might say, "I'm nervous talking about this because my mother never talked to me about it, but there are some things I feel are important for us to talk about."

Discuss your valves with your children. By talking about contraception, says Roberts, you are not pushing them into sex. You are giving them information that will help them make an informed choice when they are ready. To make it clear that it is not permission you are giving, tell him or her why you feel sexual intercourse might be emotionally harmful at their age.

Make it know that you want to hear their questions, problems and opinions. "This will add to the likelihood of their respecting you" she says. "And you might also learn something about what your child is experiencing and feeling." CAPTION: Picture, Elizabeth Roberts