But a third group is emerging from the sexual wilderness that defies current categories of gender identity. Known as transgenderists, these are biologically normal men and women - rather than hermaphrodites or those with an intersex condition - who choose to live as the third sex.
Leslie Feinberg, an American transgender activist and author, says the movement includes not only drag queens but drag kings and androgynes, bearded women and female weightlifters. "It means all the people who don't fit the narrow confines of man and woman," says Feinberg. And their numbers are growing. In cyberspace, where no one knows what's in your knickers, the Internet transgender news group has 46,000 accesses a month.
Many androgynes begin physically to alter their bodies but stop short of transforming themselves into the opposite gender: a typical transgenderist would have his (the pronoun for neuter is male) breasts and womb removed but not go on to have phalloplasty. Unlike transsexuals, they have no desire to convince the world they are male or female, but straddle the gender boundaries, challenging our most basic notions about the inherent duality of male and female. They are not trapped in the body of the opposite sex but reject the gender to which society assigns them.
Zoltar Kattse, 27, with shoulder-length blond hair and a reddish stubble, wears bright leggings and a raspberry-coloured jumper. His gender is indeterminate, as his facial hair and flat chest contradict his soft, almost feminine features. But Zoltar - a name he adopted after his double mastectomy - has travelled a long road to his current identity.
Born biologically female, Zoltar grew up acutely aware that he was set apart from other children. "I felt I didn't belong anywhere. I identified not with being a boy but with differentness." At secondary school, Zoltar felt even more bewildered when the other girls began obsessing about boyfriends, lipgloss and spots.
During puberty - "a disgusting and unnatural process" - the loneliness became acute and Zoltar contemplated suicide. He began to play truant and was committed to a psychiatric hospital. But the doctors there had a policy to ignore "gender dysphoria" (a condition where physical anatomy and gender identity are mismatched) and psychiatric treatment consisted, says Zoltar, of "trying to force me to be female".
Four years and several suicide attempts later, Zoltar chanced upon a television interview with male-to-female transsexuals. He realised that, although he felt no desire to become a man, he shared their desire for a change of identity. He found a more sympathetic psychiatrist, who put him on the male hormone testosterone, and arranged for a double mastectomy. Despite losing his nipples, Zoltar felt liberated without breasts and "wearing clothes for the first time felt right". Finally, he had found his identity.
"I realised that I wasn't male or female, but that I could be androgynous. I stopped trying to dress as a man, wearing male clothes, and changed my name to Zoltar [after an androgynous Japanese cartoon character]."
Since then, life has not always been easy. Zoltar lives with his partner Barbara Barrett, a male-to-female transsexual, on a Reading council estate where they have been the target of abuse from neighbours, who believe they are lesbians. Zoltar's gender also panicked the DSS computer, when he refused to accept the title of Miss or Mr. (He avoids the male/female public toilet dilemma by opting for the disabled loos.) But there is a more disquieting edge to the world's reaction than simply the daily inconvenience he faces. "Many English people see me as a product of everything they detest."
Other transgenderists have faced similar crises before embracing androgyny. Born female, Christie Elan-Cane, 36, appears somewhere further along the gender continuum than Zoltar, with a shaved head, flat chest and khaki shirt. Now based in London, he grew up convinced he occupied the wrong body, but also believed he wasn't a man. Standing naked before a mirror was a torturous reminder of Christie's problem, and he dieted continuously, hoping his hated breasts would disappear.
At 26, Christie began looking for a surgeon who would perform a double mastectomy. The first recoiled with horror, and even when Christie found the right surgeon, it took more than a year to raise the funds to pay for the operation. During that period, Christie began to wonder "if I was going against nature", and at his lowest moments he contemplated "getting a chisel and doing something about it myself".
Finally, Christie went under the knife. "The anaesthetist asked whether I wanted to go through with it and I said `yes', because I knew if I didn't, I'd regret it for the rest of my life. The next thing I remember is waking up again. I could feel through the bandages that my chest was flat and it was the happiest moment of my life." The second stage of Christie's operation was a hysterectomy. "I now feel happy with my body and myself - I don't have the same confusion or distress."
But living as the third sex doesn't always involve such drastic steps as surgery and a lifetime's prescription to synthetic hormones. Transgenderists who are blessed with androgynous looks often opt for living a double life, adopting the clothes and name of the opposite gender whenever appropriate. Rachel O'Connor, 39, a physicist from London, is a biological male and, at work, looks like any other man. But 15 years ago, then in an utterly conventional heterosexual relationship, Rachel embarked on a "gender expansion".
Her female lover (Rachel prefers the female pronoun), who was often mistaken for a man and even gay-bashed, became increasingly laddish, dominating conversations, bringing home rowdy friends and ignoring the housework. Rachel, meanwhile, became more housebound and subdued, increasingly bothered by "things like hair in the plughole". They parted just as their roles had almost entirely reversed.
Rachel, who defines herself as a transgenderist, finds the narrow emotional scope allowed men makes intimate, platonic relationships with either sex virtually impossible. "When I'm a man with another man, I sometimes feel stuck into a competitive aggressive situation. Men are just stunted emotionally - it's only when they're drunk that these rules can be broken."
Rachel believes she enjoys the best of both worlds by retaining her male identity at work, where she sees women losing out to men professionally. "At work I can pass as a man - I use the hierarchy to my own benefit," she says. Meanwhile, her slim figure, glossy black hair and clear skin enable her to pass as a woman at other times without resorting to surgery. She remains optimistic that society will catch up with her, as transgenderism becomes more mainstream.
But many British psychiatrists still regard the desire to live between genders with scepticism. Dr Russell Reid, a consultant psychiatrist who worked at the United States' first gender identity clinic, at Johns Hopkins University, Maryland, and now practises privately in London, believes that most androgynes suffer from a personality disorder, or are rebelling against society. "Unlike transsexuals," he says, "their gender disorder stems from socialised behaviour and their own personality, rather than what some hormone has done to their brain." He adds, however, that they are often "very unusual, very bright, very alienated, very individualistic and feel strongly that they are who they are".
In theory, androgynes explode our belief in two sexes and suggest that gender exists along a continuum, with Rambo at one end and Marilyn Monroe at the other. Dr Richard Green, consultant psychiatrist and research director at the Gender Identity Clinic at Charing Cross Hospital, London, believes we all possess varying degrees of masculinity and femininity. "It's easier for us to put everything into pigeon-holes. We don't want to think about anything that's in the middle." There are people, he says, who simply don't easily fit into our existing categories and their ambiguity can be threatening.
Living in the third gender is certainly no soft option. Dr Reid says his androgynous patients "have a terribly hard time in society - to be sitting on the fence just isn't acceptable". Outside the safe zones in the metropolis, where gender-bending is simply trendy, androgynes are still prey to gay-bashers, and the playground is a minefield for adolescents with an unconventional gender identity.
Zoltar feels he learnt about the limits of society's tolerance when he entered a psychiatric hospital at 11 and saw his childhood sliced away. Perhaps his conviction has something to teach us all about the meaning of gender.
`Sex Acts', in BBC 1's `QED' series, will be broadcast on 28 March at 9.30pm.Reuse content