Asexuality: Prevalence and associated factors in a national probability sample
Anthony F Bogaert (2004)
The Journal of Sex Research
I used data from a national probability sample (N > 18,000) of British residents to investigate asexuality, defined as having no sexual attraction to a partner of either sex. Approximately 1% (n = 195) of the sample indicated they were asexual. A number of factors were related to asexuality, including gender (i.e., more women than men), religiosity, short stature, low education, low socioeconomic status, and poor health. Asexual women also had a later onset of menarche relative to sexual women. The results suggest that a number of pathways, both biological and psychosocial, contribute to the development of asexuality.
[The female sex drive is lower than the male sex drive and, according to a survey from R. Lippa, more variable. So it's quite plausible that there are more asexual women than men. If a phenomenon like asexuality exists, there is also a phenomenon which we could call hypersexuality. It would be interesting to know the male-female hypersexuality ratio.]