Not sure if it would be a big deal. Some things are bug deals if treated as one. Transgender has been a long existing thing, the awareness of our particular civilization might not be high.
But, um, why do you assume people are clicking "other" for fun and not clicking "female" or "male" for fun? Do you have data about people's reasons for clicking? Is there some data on HN's readers' gender distribution with which you can disprove - oh wait, that's what this poll was intended to provide...
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).
"Identities"? Go fuck yourself. You don't "identify as" male or female. You ARE male or female, because you have a cock or a vag. Simple as that. Why do liberals like shit like this?
I wish it were that simple. It would save a lot of people a lot of misery. Unfortunately we organisms are unimaginably complex, and it turns out that, although the vast majority of the time it comes down male/female, things do land on a spectrum in terms of biological sex.
(heterosexual male upper-middle class white American here...)
EDIT: Added it on public demand.