Context: When discussing with Gender Propagandists who claim over 100+ genders exist, they always tell us that biological sex isn't a binary either. This is far from the truth, however they ask us to define what it is, so this short post will answer them:
The best way to define it is based on the body's attempt to create gametes. Some people can't create gametes, but their body is based around ovaries (female gametes) or testes (male gametes). In most cases chromosomes dictate which gamete you can produce. There are some cases of intersex where that isn't the case, but in the end, their gamete production determines the sex. Chromosomes are accurate for 99%+ of people. Attempted gamete production is accurate for every living person. And it always falls into male or female, except for "true hermaphrodites" who are potentially both male and female. However, it is also possible even true hermaphrodites can only produce one gamete. No true hermaphrodite has both fathered and mothered a child. But some have fathered a child and some have mothered a child. So, thus far in human history everyone has been either male or female as far as we know, not both.