Fish, in particular, have been well studied. When a school of some reef fish loses its single male, the largest female can begin to change into a male within hours, producing sperm inside a few weeks. Such sex changes are fairly widespread taxonomically. Individuals from species in at least 14 families of fish can convert from female to male, and conversion from male to female occurs in eight. Some switch back and forth during a single mating episode.
Sex can be temporal.
Thus our lives may not be the best models for understanding the complex nature of sex. Even among humans, though, sex-distinctions are not unequivocal. The variations may not surprise someone familiar with embryology. Every mammalian fetus, after all, has structures that can develop into a uterus and fallopian tubes, as well as those that can mature into the sperm-transport system. But what is more surprising is the frequency of such intersex individuals at birth.