What Other?

Last week, I received an e-mail about a survey regarding the student union at the campus where I go to grad school. (I am not sure which building it is in, but they were offering prizes for completing the survey.) The survey was set up and run by an outside organization with many standard questions. There was a page of personal information such as age, academic standing, marital status and so on. Then I came to the part about gender (which should really be sex, but we cannot say sex because it is a "dirty" word). Normally, one sees two options at this point, and they are pretty straightforward: male and female. It is not quite like race, where one can have ancestors from more than one race. Under gender they had male, female, transgender and other. Male and female I get. Transgender? Huh? Even if one accepts the idea that a person could be transgender, what in the world would classify as other? Is there some sort of evolutionary process at work here that I am not aware of? I thought about clicking "other" just for kicks, but I stuck to what I have put on every form previously.