EDIT: Added it on public demand.
You should just add another option and ignore the haters.
(edit: thanks)
Now I'm curious about the fraction of "other"
I quite doubt there are 15 other to 42 female (to 735 male).
Other than http://blog.xkcd.com/2010/05/03/color-survey-results/ and specific gender/sex studies, I haven't seen studies that make a distinction between gender and sex. Of course, if you want to make the distinction about biological sex, then you have to define which of the biological criteria and categories you're using, and even then you need more than two categories (http://linuxmafia.com/faq/Essays/marriage.html).
You seem to suggest that my interpretation is arbitrary, while I think it is the most colloquial. Do you honestly think that a 15 year old girl/boy would see this thread and think "well I am neither of those"? Or are you just deconstructing a very straightforward question by its edge cases to prove some kind of point that I'm missing?
Sorry it's such a big deal to you.
(heterosexual male upper-middle class white American here...)