It wasn't until Roberts was in her 40s, and in a relationship with a woman, that she concluded she was asexual - simply not interested in sex.

"She was not happy to hear that," Roberts, a 48-year-old Mountain View resident, said of her incredulous ex-girlfriend. "She equated sex with love. I think that was true for her, but not for me."

The relationship ended last year, but Roberts had emerged from years of confusion. And in June, while wearing an "Asexy Dyke" T-shirt, she marched with two dozen other self-described asexuals in San Francisco's Gay Pride Parade.

It was an unusual coming-out for people who consider themselves members of a fundamental sexual orientation. At an event that celebrated sex of every flavor, one group declared an intention to skip the buffet.

The movement is a testament to how vital sexual identity is to self-worth. But the public emergence of asexuals also raises questions for sexuality researchers - mysteries involving the fluidity of sexual identity and the link between romance, which many asexuals enjoy, and sexual attraction.

'We're not broken'

"It does raise questions about the nature of love," said Anthony Bogaert, a sexologist at Brock University in Ontario who estimated the prevalence of asexuality in 2004. He analyzed an earlier survey of Britons and found that 1 percent reported that they had never felt sexually attracted to anyone.

David Jay, a 27-year-old San Francisco resident, put the movement in more personal terms, saying, "We need to know we're not broken. I've been told my whole life that people need sex to be happy."

The Pride Parade was a milestone for Jay, who is studying for his graduate business degree at Presidio School of Management. Nine years ago, he essentially started the movement by founding the Asexual Visibility and Education Network, or AVEN, as a teenager who couldn't fathom why everyone but him was hell-bent on shedding their virginity.

Seeking respect

Jay and his online community, which he said has 30,000 registered worldwide members, aren't seeking to create new civil rights. What they want is respect in a sex-obsessed culture.

Asexuality has only occasionally been studied, but the few researchers who have given it a close look in recent years say it may be a sexual identity similar to being straight, gay or bisexual.

Dr. Lori Brotto, an expert on sexuality at the University of British Columbia in Vancouver, said she was once "extremely skeptical" that asexuality existed as an orientation. But in 2007, in surveys of AVEN members, she found not only low sexual desire but low distress about it.

"They're not bothered by the low levels of arousal," Brotto said. "That's what makes them different from someone with sexual dysfunction, who wants to seek treatment."

Recently, Brotto showed erotic films to seven asexuals along with 35 other women who identified themselves as straight, lesbian or bisexual, while measuring vaginal blood flow. She found no physiological differences in their responses.

"That's kind of what we predicted," she said. "This is not a sexual dysfunction. It's a sexual orientation issue."

Some asexuals are romantically straight, gay or bisexual, and some aren't romantic. They date each other, or they go out with "sexuals," attempting to compromise in bed.

Roberts, a software engineer, said she's worried she won't find somebody with whom to grow old. "For me, they have to be asexual and lesbian," she said, adding with a laugh, "Then there's that whole compatibility thing."

No common culture

The birth of the asexual movement has been as tricky as the personal stories, involving people whose sexuality is naturally passive. They lack the cultural markers claimed by the gay community - styles of dress, for instance, or bars in which to gather.

Into that breach came the great uniter of obscure groups - the Internet - and Jay, whom many asexuals consider to be an ideal spokesman. Young, charismatic and good-looking, here is a man, they say, who could have sex if he wanted to.

Speaking over burritos at a taqueria near his Mission District home, Jay said he was driven by memories of feeling alone. As a teenager in St. Louis, he searched the Web for "asexual" and found only research on amoebas.

He launched AVEN and recalled that when the first kindred soul got in touch, "we had this intense, two-hour discussion, going into all these things no one else could relate to."

Jay likes to say that nonsexual relationships are as rewarding - and challenging - as sexual ones. In defending asexuality, he is unflappable, even when being used for comic relief.

On MSNBC, host Tucker Carlson asked Jay if he was gay and repressed and wondered why, with all his free time, he hadn't cured cancer.

"If I'm able to define asexuality every time I do an appearance," Jay said, "we get 50 to 200 new people coming to us."

There are other signs of momentum. Bogaert is writing a book about asexuality, a New York film company has a documentary about the subject in the works, and in New Zealand, a soap opera features television's first identified asexual character.

AVEN members have one concrete goal: changing the authoritative Diagnostic and Statistical Manual of Mental Disorders to make explicit that asexuality is not a "hypoactive sexual desire disorder." The next edition will be published by the American Psychiatric Association in 2012.

Trying out intimacy

The primary focus of the movement, though, remains reaching people like Bridget Rodman, a 19-year-old San Francisco State student.

Rodman said she gets crushes on women but is not driven to sleep with them, which for years made her feel useless, even suicidal. She recalled trying to "mimic the patterns" of a girlfriend during an intimate moment.

Then, in November, she found AVEN's Web site. It described her so accurately, she recalled, that she cried over her keyboard.

But with the discovery came revelations: that she could be honest with people, that she could experiment with intimacy as long as her partner was willing to stop, and that she no longer had to wait to live fully.

"It's indescribably amazing," Rodman said of the change. "I can build my own ideas of what I want to be instead of waiting for this biological lubricant, literal or figurative, to come along."

This article has been corrected since it appeared in print editions. E-mail Demian Bulwa at dbulwa@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle