Not very much is known scientifically about asexuality, although thanks to the internet we're learning more about the men and women who describe themselves as asexual. These individuals maintain that they have no interest in sex, feel no sexual desire, have no sexual fantasies or dreams, and appear happy to be that way. The sparse research on this topic does include an analysis of data gathered by questionnaire from Britain, which showed that of the more than 18,000 men and women who participated in the survey, one percent identified themselves as asexual (three percent identified themselves as homosexual). The analysis found that 44 percent of the self-described asexuals were married, living with partners or had been in the past. Asexual respondents were more likely to be women than men in the British study, but in more recent research among college-aged, self-identified asexuals, the gender breakdown was fairly equal.
Based on what I've read, there is a big difference between asexuality, which is lack of interest in sex, and celibacy, which is a choice not to engage in sex. Asexuals say that they're not attracted to men or women and have no desire to have sex with either gender.
So far, no hormonal abnormalities have been associated with asexuality, and low sex drive or hypoactive sexual desire disorder does not appear to play a role. (People who seek treatment for this disorder are generally pretty unhappy about it.) Some people have an aversion to sex because of a childhood trauma, but as far as we know, that's not typically the case here. Other than their lack of interest in sex, asexuals seem to be normal and healthy, with no underlying genetic glitch that might explain their lack of desire.
While some asexuals claim to desire romantic relationships, they say they're simply not interested in the sexual aspect but might engage in sex to satisfy their partners. I've also read that some asexuals masturbate but claim not to arouse themselves via sexual fantasy and not to derive pleasure from the act, which raises the question, "Why bother?"
Most of what we know about asexuality comes from the stories of the men and women who define themselves that way. What appears to have changed in recent years is the ability of these people to connect via the internet and websites that provide forums for discussion and (non-sexual) connections. Clearly, this is an area that is ripe for research. Lacking evidence, it is hard to say whether asexuality is a fourth sexual orientation (after hetero-, homo-, and bisexuality) or something else.
Andrew Weil, M.D.