Finally got around to looking at this one, and I have to say I am a bit stumped by the choice of question.
The number of genders recognized in any given society is an anthropological question: Wikipedia entry on Third Gender
WP
I have to assume that the respondents were not asking themselves whether the society they live in recognizes third/fourth/fifth genders (Western societies lack a third gender role unless you count the emergent idea of non-binary individual to whom no gendered norms apply) but rather whether they want a third category added to the existing sets of social norms associated with masculinity and femininity.