SEXISM IN GENRE PUBLISHING: A PUBLISHER'S PERSPECTIVE « Tor Books UK: "The sad fact is, we can’t publish what we’re not submitted. Tor UK has an open submission policy – as a matter of curiosity we went through it recently to see what the ratio of male to female writers was and what areas they were writing in. The percentages supplied are from the five hundred submissions that we’ve been submitted since the end of January. It makes for some interesting reading. The facts are, out of 503 submissions – only 32% have been from female writers."
As I just noted at Science fiction's real-life war of the worlds | Books | theguardian.com: "Some genres have different gender stats. I would never argue that the Romance field was sexist simply because more of its readers and writers are female."
ETA: There's a bit of discussion about this on Google+.