The Fairfax County School Board, putting aside concerns that it might be fostering promiscuity, homosexuality, abortion and amorality among teenage students, last night approved lesson plans that will revamp its decade-old sex education program.

The unanimous vote was the last major stage in approving a top-to-bottom overhaul of sex education in Fairfax as required by the state in 1988. Last June the board agreed to the broader program outlines, and this spring specific curriculum was tested in several schools.

"I think it's high time that we provide an appropriate family life program that's appropriate for the 1990s," School Board Chairman Kohann H. Whitney said.

Parent critics said the move will introduce "value-free" sex education that will encourage teenagers to experiment sexually, possibly exposing them to AIDS.

"What you end up having is a system of children who are risk-takers, who experiment, who make their own decisions regardless of what their parents taught them," said Ellen Shepherd, a leading opponent who plans to remove her five children from Fairfax schools this summer. "This program affirms illegal, immoral acts."

The lesson plans approved last night will replace substance abuse prevention materials in grades 1-5 and 7-9, rewrite the human life unit in 10th grade and create a new four-week family life unit for ninth-graders. Among a wide variety of topics to be covered in that ninth-grade unit will be contraception, abortion and homosexuality, the latter for the first time in Fairfax.

Critics, including a grass-roots group of parents calling themselves the Fairfax Citizens Council, strongly complained that the lesson plans present delicate information about sexuality without any moral guidance.

For instance, some parents see a contradiction in discouraging pre-marital sex and then outlining various methods of contraception. Others object to presenting homosexuality in a neutral fashion, saying that approach ignores predominant Judeo-Christian values and might encourage confused students to become gay.

While parents can opt to keep their children out of the lessons, some said schools have made it extremely difficult to do so.

Shepherd said she and other parents will opt out altogether and form their own private school that will teach "traditional values."