The Fairfax County Board of Supervisors, three months from the fall elections, came under intense pressure from Christian activists yesterday on three issues central to the activists' agenda: abortion, topless bars and a religiously oriented day school.

The board rebuffed the avowed conservatives on all three issues, turning a cold shoulder to a proposal to prohibit abortions at Fairfax Hospital, resisting demands to shut down the county's handful of establishments that feature seminude dancing and denying a special zoning exception for a proposed Christian school.

The conservatives, some of them warning of a "flood of immorality," scored a partial victory on the nude-dancing issue when the supervisors voted 6-to-0 to restrict the opening of new topless bars and strip joints. But the board declined to follow the lead of Prince William County in banning the establishments, as some Christian and conservative leaders had urged.

In another vote, the board rejected for the second time in two years a proposal from Fairfax Christian School to build a 576-student campus on a wooded 42-acre site in the Oakton area. The application for a zoning exception was defeated on a 5-to-4 vote. The proposal, which drew heavy fire from nearby residents, became embroiled in further controversy because the school's owner, Robert L. Thoburn, is an independent candidate for the Board of Supervisors.

In the third defeat, the supervisors, who had been angrily shouting in a closed-door session, ended that session by directing County Attorney David T. Stitt to respond to Neil F. Markva, a prominent Northern Virginia conservative leader who advocates eliminating abortions at Fairfax Hospital. Stitt has said the county has no such authority.

The votes on nude dancing and the Christian school followed impassioned pleas from a parade of speakers.

"Having topless bars in our neighborhood is morally degrading, as well as running down the quality of the area by bringing in individuals with prurient interests to patronize them," said Marie A. Kuhn, a Springfield resident. "It is like having part of 14th Street in your own neighborhood, and I'm sure you will agree with me to let Mayor Barry keep 14th Street."

Fairfax County Board Chairman John F. Herrity, who had championed the seminude-dancing restrictions, said Stitt had advised the board that it lacked legal authority for a ban on the establishments. He said the new regulations -- which require the board to give special exceptions to strip joints and topless bars and limit them to regional shopping centers -- represent, for practical purposes, a ban on all new establishments.

The defeat of Christian Day School drew acid responses from many conservatives, who charged that the proposal had become a political football because of Thoburn's independent challenge to incumbent Republican Nancy K. Falck in the county's Dranesville District.

"You know and I know that there's been a hell of a lot of Christian bashing," said retired Brig. Gen. Benton K. Partin, a former chairman of the county Republican Party and an outspoken conservative Christian. "This is just another case of Christian-bashing,"

On the abortion issue, Herrity argued that Stitt should respond to Markva, who wrote to the chairman that under the terms of Fairfax's lease with the Fairfax Hospital Association, the county could stop abortions from being performed at the hospital. Doctors at the hospital performed about 265 abortions in 1985, according to hospital spokesman Lon Walls.

Markva, who has ties to the Rutherford Institute, a conservative organization in Manassas, took issue with a memorandum drafted earlier this year by Stitt. In the memo, dated Feb. 10, Stitt contended that under the $1-a-year lease, county officials had no control over the operations or policies of the hospital, short of terminating the lease.

"Performing abortions at the Fairfax Hospital does not violate any provision of the lease," Stitt wrote.