I don't like to get into the sex vs provider argument because sometimes I feel it doesn't fit and it's a bit simplistic.
Emjay mentioned the study where it was women who were looking at the sex bits, not the men. Here's an article on it:
http://www.msnbc.msn.com/id/19330231/
The statistic I always hang my hat on is Neilsen Netratings finding that 30% of all porn surfers are women. Not as much as men, but plenty significant.