Poston, D. L., & Baumle, A. K. (2010). Patterns of Asexuality in the United States. Demographic Research, 23, 509-530.
Abstract: In this paper we use data from the 2002 National Survey of Family Growth (NSFG) to ascertain and analyze patterns of asexuality in the United States. We endeavor to extend the earlier work of Bogaert (2004) on this topic, which focused on patterns of asexuality in Great Britain. Using a social constructionist perspective to study asexuality, we conceptualize and measure the phenomenon in several ways, according to behavior, desire, and self-identification. We use the NSFG respondent sampling weights to produce several sets of unbiased estimates of the percentages of persons in the U.S. population, aged 15-44, who are asexual; each set is based on one or more of the various definitions of asexuality. Finally, we describe some of the characteristics of the asexual population using logistic regression.
It is in an open access journal, so you don't have to pay anything to read it (or need a library with a subscription).