Are There More Than Two Sexes?

 City Journal: Colin Wright follows the science. An excerpt:

When biologists speak of sex being “binary,” we mean something very straightforward. There exist only two sexes, which are fundamentally rooted in the binary classification between sperm and ova. Males have the function of producing small gametes (sperm), and females large gametes (ova). Other measurable sex differences beyond gametes (which include genetic differences, hormone levels, and average morphological and behavioral differences) are either a cause or consequence of this fundamentally binary and definitional distinction between males and females, and they need not be binary. Put simply, not all sex differences are differences of sex.