What in your studies of biology has taught you that human societies recognize two genders?
There are animals for example that have 2 sexes but more "genders" (to the extent that it is correct to apply this terminology) - e.g. Ruffs (a european bird) have several male forms that differ in size, plumage and sexual behavior ( https://en.wikipedia.org/wiki/Ruff#Biology_of_variation_amon...).
Even if no similar mechanism exists in humans, there are social constructs that may serve similar roles - there are communities of "3rd gender" people in India for example, for thousands of years now.
There are animals for example that have 2 sexes but more "genders" (to the extent that it is correct to apply this terminology) - e.g. Ruffs (a european bird) have several male forms that differ in size, plumage and sexual behavior ( https://en.wikipedia.org/wiki/Ruff#Biology_of_variation_amon...).
Even if no similar mechanism exists in humans, there are social constructs that may serve similar roles - there are communities of "3rd gender" people in India for example, for thousands of years now.