While signing up today I was glad to see a gender option other than male and female. However, transgender is not generally regarded as a gender identity in and of itself, at least not in the same way that male and female function. Transgender is used as an adjective rather than a noun; so, someone might define their gender identity as "transgender male" or "transgender female," with transgender as a modifier of "male" or "female." So a trans man or trans woman who signs up will have to choose between the two applicable aspects of their gender identity, while others (like me) still won't really have anything that fits.
Of course, since breaking away from binary gender (which I'm glad you did) eventually yields a multitude of choices that are difficult to easily track with a radio button field, there's no easy solution. Some ideas:
a) The "Gender" field can have three options: female, male, other. You can then have an additional field (not sure what it would be called) with these some or all of these options: trans, non-trans/cisgendered, genderqueer, other.
b) You could have a Gender field with some or all of the options above, and use checkboxes instead of radio buttons so that people can select all that apply to them.
c) Something better/smarter that someone other than me thinks of!
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