The crucial distinction that must be made here is the difference between sex and gender. Sex is a biological term referring to the chromosomal makeup of an individual; gender refers to the characteristics associated with those sexes. It is reasonable to say that there are as many genders as people.
A classification of gender is a classification of a generalised societal role. Generalised societal roles derive from the needs of a society. These needs of a society are demanded by a specific biological function. Rejecting a generalised societal role is rejecting social gender classification.