@lukeshu @mjg59 @lxo ok, being completely real, that's only the later reason, the -original- reason, at least in the modern sense of competitive sport which basically runs from late 19th century, is that it was unseemly for women to compete at all.
But my main point is that "fairness" drives the need for categories, but the definition was created without looking closely at the edges of those groups. Humans are fuzzy. Drawing straight lines will -always- result in places that don't fit.