[john_b]

If you are in SF, this is partially due to how the tech industry distorts the sex ratio there:

http://pics.city-data.com/agegraph/2455.png

(This is for residents of SF only, so it won't account for any differences that might arise due to commuters who live outside SF)

Edit, source page: http://www.city-data.com/housing/houses-San-Francisco-Califo...