The following were among actions taken by the Fairfax County Board of Supervisors at its Dec. 14 meeting. For more information, call 246-3187.

SEX EDUCATION -- The Board of Supervisors voted unanimously to oppose Virginia Board of Education plans to require sex education in the state's public schools.

In an earlier 5 to 4 vote along party lines, the board's Republican majority defeated a substitute proposal by board Democrats to study the state plan before deciding whether to support or oppose it.

The state Board of Education voted Dec. 9 to require all public schools in Virginia to teach a "family life" curriculum that would include information about contraception, homosexuality, human reproduction and sexually transmitted diseases.

Local school systems are to have some discretion in deciding when and how to teach these subjects, but would have to include them somewhere in their curriculum or lose state matching funds.

The board resolution calls on the county's representatives in the General Assembly to support continued funding for Fairfax County's own sex education program.

Board Chairman John Herrity, a Republican, introduced a resolution that the board should "resist" the state-mandated program.

Herrity said that Fairfax County already has its own sex education program and said such a curriculum "with all the emotional, social and ethical aspects of its subject . . . must be a matter of local option."

Although Supervisor Martha Pennino (D-Centreville) voted for the final resolution opposing the state plan, she criticized Herrity's resolution. "It's a School Board issue and that's where it belongs," she said.

Supervisor Katherine Hanley (D-Providence) said that Fairfax County's present sex-education program may meet the new state requirements anyway and that the new state plan could give the county more money for course materials. She proposed a substitute to Herrity's measure, referring the issue to the School Board and asking the school staff to study how the state-mandated program would affect the county. Her substitute failed, with the four Democratic supervisors voting for it and the five Republicans voting against it.

The supervisors then agreed to modifications in Herrity's proposal suggested by Democrats. They changed Herrity's original suggestion to "resist" the state-wide program to a call to "oppose" it. They also added a request that the school staff study the state plan they were opposing.

MIDYEAR BUDGET REVIEW -- County Excutive J. Hamilton Lambert suggested midyear changes in the county's current $2 billion fiscal 1988 budget, which began July 1, proposing $6.8 million in new programs and deferral or reductions in other expenditures to compensate for a decline in tax collections.

The two major new spending recommendations are $2.6 million for 584 new electronic voting machines and $1.1 million for extra pay for county firefighters.

Lambert said sales taxes and personal property tax collections are below original projections, largely due to lagging car sales and a drop in other consumer spending. He proposed deferring funds for some programs, including $1.2 million to expand the Clifton fire station, $1.2 million for a new pedestrian trail near Annandale Road and $3.7 million for a new county garage in Centreville. The board will hold a public hearing on the proposed budget changes Jan. 14.