That's a lot of people you're being surprised at.
I couldn't find any epidemiological studies, but the reality is that porn is a very successful business venture. That business has got to be going somewhere.
Pornography and Erotica: Definitions and Prevalence
Nigel K. Ll. Pope, Griffith University,Kevin E. Voges, University of Canterbury,Kerri-Ann L. Kuhn, Ellen L. Bloxsome, Griffith University
It has been estimated that the global sex industry generates between $30 billion and $50 billion per annum in sales (Hughes, 2000). In 1983, this segment was estimated to produce annual revenue of $8 billion in the United States alone (Cowan et al., 1988; Pornography Resource Center, 1984). By the turn of the century this at least doubled (Lane, 2000). In terms of distribution and penetration, the American industry produced 10,000 feature films in 1999 (Slade, 2001), and constitutes about 14% of the video rental and sales business, as well as more than half of the pay-per-view hotel video market (Economist, 1997). Americans spend in excess of $4 billion per annum on pornographic and erotic videos and DVDs (Egan, 2000). With regards to novels, 26 erotica titles had sales of more than 11,000 copies each in 2005 (Patrick, 2006), prompting some of the big romance novel publishers to now diversify into this area (Dang, 2006). In 1996 on the World Wide Web there were 5,000 commercial pornography/erotica sites operating in the USA, and by 1999 there were 30,000, generating between $150 and $200 million per annum for the most successful (Morias, Nelson and LaFranco, 1999). It is alleged that 43% of Internet traffic goes to a sexually explicit site (Tedesco, 1998). In fact, these sites apparently represent the most frequently visited online, particularly for young males aged 15 to 25 who use the Internet as their primary source of pornography (Häggström-Nordin, Hansson and Tyden, 2005; Wallmyr and Welin, 2006).
etc.