The point is that "gender" is a grammatical term. "Sex" (a word that the sex-obsessed deviant community seems oddly prudish about) is a biological term, and there are two sexes: male and female.
Yes, occasionally the genetic wires get crossed and produce hermaphrodites; yes, individuals of some species (in what used to be called the "lower orders") can swap from one sex to the other; yes, some other species (certain plants and invertebrates, and a very few animals) reproduce asexually; and yes, some (plants and crustaceans) alternate between sexual and asexual reproduction.
However, last I looked, people aren't plants or molluscs (although certain politicians' conduct is strong evidence for the existence of mammalian invertebrates). The point, as posters have noted, is that in human biology, sitting somewhere in the middle of the hypothesized continuum between male and female doesn't imply the existence of a third sex, and for damned sure doesn't mandate its creation by fiat.