OK, I'm gonna try something ....
Et voila! (give or take a few accent marks) -- a homemade gender field!
All I did was add the information to my profile's Location field, which now contains:
Indiana, USA Gender: Female
The key is to make the actual location info just long enough to fill the first line, so that the rest of the field (in this case, "Gender: Female") wraps around to the second line, so that it looks like a separate field.
Added: Unfortunately, color codes do NOT seem to work within the Location field. I tried making mine pink (or actually violet, which is the closest available), but all it did was display the BBCode along with my info!
Note that the Location field is much longer than it looks. If your actual location info is longer than one line, you could probably get a third line's worth in there for your gender.
And note that you have to enter your password in ALL THREE places in order to change your profile.