The responses above all point in the same direction...

Camera stores, camera shows, and even forums like APUG are primarily about camera gear and the technical aspects of photography --> they attract an audience that is majority male. This then reinforces itself - I imagine a number of women drop by APUG and then leave again when they see the almost exclusively male population here.

Photography/darkroom classes with an emphasis on art and creation (rather than just gear and technique) tend to balance things up by attracting the women back again.