I could be wrong, but I don't believe Shiina Ringo's music attracts more males than females, especially not within Japan. I can find plenty of blog entries from random females praising Shiina Ringo or at least listing her as an interest, and I'm sure the results that turn up at Last.fm aren't so slanted toward the males, if/when you can even gauge gender at all.
The audiences we can see in the concert DVDs seem to be fairly gender-balanced, or maybe that's because in Japan, every girl who listens to Shiina Ringo has a boyfriend, or vice versa.
(at least as far as heterosexuals go - because I can't speak for the rest)