Gender IS in preferences. You can choose between male, female and unspecified. This is to customise UI on the site so it has the appropriate pronouns. You can do this all over the place, like this:
{{gender:Jimbo Wales|man|woman|person}}
(replacing Jimbo Wales with your WP username (or a template that substitutes the current user's name) and the words man, woman and person with wikitext that you want returned)
This is used quite a bit for Userboxes so that they can make it text in the userbox switch dynamically between "This user lives in London and [they like/he likes/she likes] travelling on the Underground" or whatever.
The problem with the preferences route is unspecified may be because you haven't set it or it may be because you don't want to set it (or you don't fall into male/female because you are transgendered or whatever).
There have been polls and studies done though. You can read about them on http://enwp.org/WP:BIAS or http://enwp.org/WP:ACST