Ever since the 1953 Kinsey Report, conventional wisdom has held that 10% of the U.S. population is gay. Forty years later, the Janus Report stated that 22% of American men had had a homosexual experience, but last week the Alan Guttmacher Institute reported that figure is only 2.3%.

Likewise, the public has heard conflicting reports on infidelity, the practice of safe sex and the prevalence of rape.

Does anyone really know the truth?

Knowledge about sexual behavior has become an urgent matter in the United States, with widespread concern over AIDS, abortion and teen-age pregnancy. But while sex surveys have become more common, sophisticated and accurate than ever before, the art of turning intimate acts into scientific data remains . . . delicate.

Today researchers can ask difficult personal questions that would have been unthinkable in years past, such as "Have you had anal intercourse? With men? With women?" But they acknowledge such questions can also skew a survey by either repelling or attracting participants, or embarrassing people into fudging.

"You always wonder: Are you getting over-reporting, or under-reporting?" said Kristin A. Moore, executive director of Child Trends, a Washington, D.C.-based research organization. With the exception of questions regarding abortion, which can be independently verified, "you don't have a way to tell."

Scientifically valid studies of sexual behavior, with samples that represent all groups, did not begin until the mid-1980s. So far, there have been only a half-dozen "good studies," said Tom Smith, director of the General Social Survey at the University of Chicago's National Opinion Research Center. The General Social Survey has been tracking trends in American society since 1972.

Pioneer sex researcher Alfred Kinsey published "Sexual Behavior in the Human Male in 1948" and "Sexual Behavior in the Human Female" in 1953 based on face-to-face interviews with 5,300 men and 5,940 women, all Anglos, in the United States and Canada chosen by "quota sampling and opportunistic collection."

The "Janus Report on Sexual Behavior," conducted between 1988 and 1992 by sex counselor Samuel Janus and his wife Cynthia, a physician, used written questionnaires and 125 interviews with 2,765 men and women, chosen to approximate a cross section of the United States.

In 1991, the Alan Guttmacher Institute's National Survey of Men studied 3,321 men, ages 20-39, in a population-based, nationally representative survey.