The Fairfax County School Board decided last night to initiate a study of its two-year-old sex education program to detemine whether it should be revised or expanded.

The primary purpose of the review will be to determine whether homosexuality, contraception, abortion and masturbation should be added to the sex education cirriculum. The county's program now exludes these topics, concentrating on physiology and the reproductive cycle.

No deadline was given for the study, which was initiated in reaction to publication last year of Virginia state guidelines for sex education programs which are optional for local school systems. Those guidelines outline procedures for teaching the four topics now banned from the Fairfax curriculum.

Although the proposal for the study called for reviewing the Fairfax program at all levels, the board decided to limit it to the high school level because the public has expressed most concern over sex education in high schools.

Board member Robert E. Smith said he wanted to "make it clear to the public . . . that this is not an effort on the board's part to put something over on the community."

He stressed that there will be a high degree of community involvement in the study and that it "does not preclude the possibility that no changes will be made" in the curriculum.

In other action, the board reelected Rodney F. Page to a fourth term as chairman. Page has served on the board since 1974.

Ruth H. Dell was elected to her first term as board vice chairman, replacing Anthony T. Lane who held the post for the last three years.