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...
You should just add another option and ignore the haters.
(edit: thanks)