Anne Fausto-Sterling

Different organisms have different sexual systems. And sometimes there's two, sometimes there's more than two.

Sometimes the same individual animal changes sexes during the course of the life cycle. And this has been perfectly well understood by biologists for a long time.

They'll live in a big school of fish with a dominant male and a group of females. And if the dominant male dies, one of the females literally transforms into a male. So technically, that's still two sexes, but the sexes aren't fixed for life.

Biological sex.

You can think of a model in which there is there's only two. and they completely don't overlap. You always know which is which, no matter what measure you're using, whether you're looking at the genitals or the chromosomes or the gonads or the hormones. And the fact is that that model doesn't exist in nature at all.

The answer to the question, are there always two sexes, is no.