I think that observation goes for any physical activity, and life in general IME. I only know 2 female motorcyclists (who actually ride, not just sit on the back of the bike), and when I am riding around I will see 15-20 males to 1 female rider. Same thing with paintball, hiking, 4 wheeling, and geocaching. The ratios do vary between them, however.
I wouldn't be surprised if it was due in some part to cultural influence, especially in my generation. Most females I know are more interested in going to the club and getting their groove on, partying, or just staying indoors vs. getting out, getting dirty, and having some real fun.
That being said, Japan I swear has almost no male divers. There's a company here on Oahu that caters to the japanese tourist crowd, and every time their bus pulls up to a dive site about 8-9 japanese women jump out, and no dudes. Except maybe the instructor. My OW instructor said that most of the divers in Japan are female, but I don't know about the grounds for his statement.
Peace,
Greg