Arlington County Board members, heeding pleas from leaders of the county's gay population, voted yesterday to prohibit discrimination based on sexual orientation, becoming only the second Virginia locality to have such a law.

The vote, which came more than three years after the county refused to add sexual orientation to its original human-rights law, was greeted by a long standing ovation by about 100 gay-rights activists crowding the chamber.

"Obviously, it's a very important day for all of us," said Lee Stinnett, former president of the Arlington Gay and Lesbian Alliance, who was the group's spokesman. "It's an important first step in helping all parts of this great community to take a stand on this issue."

The new law prohibits discrimination in a range of areas, including employment, housing, public accommodations, credit and education. Similar laws exist in the District, Alexandria and Montgomery County.

Arlington's ordinance enables the county's Human Rights Commission to investigate allegations of discrimination and, if needed, to seek a court order securing compliance from anyone found guilty of discrimination. The law would exempt religious organizations and landlords of many small rental complexes.

The vote, which was unanimous with board member William T. Newman Jr. (D) absent, followed an emotional debate in which some gay speakers recalled incidents of discrimination and opponents raised legal and religious objections.

"A person's sexual orientation shouldn't matter. It should be a non-issue," said Jay Fisette, director of Whitman-Walker's Northern Virginia AIDS clinic. "The simple fact, however, is that there is a need. Discrimination is real. It hurts."

Helen Blackwell, a former county Republican chairman, said opponents like herself were not motivated by prejudice. "If you assume that I am hate-filled and bigoted, you are yourself biased," she said.

She argued that homosexuality is a choice, and not an unchangeable condition, and therefore did not merit legal protection. Several gay speakers rejected that description, saying their sexual orientation is inborn.

Other opponents said the ordinance would violate Virginia state laws, including the Dillon rule, which prevents local jurisdictions from claiming powers not specifically granted by the state. Some also said that homosexuality violates religious principles and should not be put in the same class as race, sex, national origin and religion in anti-bias legislation.

Concerns about its legal authority caused the county to omit sexual orientation from its original rights ordinance, but lawyers representing the Arlington Gay and Lesbian Alliance convinced the board that it was permitted to take the action under the county's general policing powers.

In other action, the board set a $400.4 million target for its fiscal 1994 budget, which would represent a 6.1 percent increase over this year's budget. That would create a revenue shortfall of $14.5 million and virtually force the county to raise its property tax rate for the second straight year.