The hearing held in an intermediate school near Falls Church last night was the last chance for the 87 speakers and their hundreds of supporters to let the Fairfax County School Board know exactly where they stand on a subject that stirs them deeply: sex education.

"Sex does not have to be taught. Sex is a natural instinct," declared Mary Lou Curtis, who not only asked the board not to expand the current sex education program -- known as the weakest in the Washington area -- but asked total withdrawal of the topic from the school system. "Chastity, virtue, purity of body need to be instilled in the minds of our children. . .

"Fornication is breaking a Virginia law!" Curtis shouted as she ran out of time and her microphone was cut off.

As it has been in the last five years when sex education has become an issue that deeply divides the county, the views given the board members last night were sharply opposed. In contrast to past such hearings, opponents of expanded sex education did not heavily predominate, although they appeared to have a majority of speakers.

Previous attempts to liberalize the program were barraged with opposition by conservative and religious groups, but this year proponents of an expanded curriculum have taken encouragement from a school system poll that said 75 percent of county parents favor such a move.

Mary Ann Beall, speaking for the Northern Virginia National Organization for Women, said her organization "stands uncompromisingly for sex education in the public schools on all levels. Our children must be prepared to deal with the basic facts of their lives, sexual realities and social pressures among them." Other supporters of liberalization argued that it would help reduce what they called the county's "skyrocketing" teen-age pregnancy rate and the incidence of venereal disease among the young.

Many opponents argued, as did Mary Finnerty of Virginia Right to Life, that an expanded program would encourage sexual activity among the young by teaching it in the schools. The opponents, who included representatives of conservative Christian churches, asserted that teaching sex education in the absence of strong moral guidelines would destroy the morals of schoolchildren. A representative of Bishop Thomas Welch of the Roman Catholic Diocese of Arlington, expressed opposition to an expanded program.

The present "family life curriculum," as sex education is called in the county schools, allows no mention of six "taboo topics": abortion, birth control, homosexuality, masturbation, rape and incest. No oral questions are permitted. Students must submit questions on 3-by-5-inch file cards.

The proposal before the board is for a program that would include material on birth control and allow open discussions of any topic.

The board is scheduled to vote on the program next Thursday night.

Last night's hearing at Luther Jackson Intermediate School, which lasted five hours, was more subdued than similar sessions in the past -- there was no shouting, for one thing. The opposing sides tried to outdo each other with the loudest applause for their speakers. Three persons who sat at the front of the school auditorium conspicuously displayed red plastic roses as emblems of their opposition to abortion.