Is there a principled reason why men and women compete separately in sports? Presumably it allows women to compete who wouldn't stand a chance otherwise. But there are many groups of people who don't stand a chance against the world's top athletes. We don't have a separate Olympic division to accommodate non-African sprinters, for example, in addition to non-male ones. So why is sex the relevant way to categorize people here?
(Disclaimer: I really don't know a thing about group differences in athletic performance, and don't care enough to fact-check. This post is based entirely on the stereotypes I've heard. If untrue, amend as appropriate. Or just imagine a possible world which is as described, and consider how our ethical principles would apply in such a case.)
Overweight people would be an even more obviously disadvantaged group, though perhaps the thought is that they could have become world-class athletes if they just tried hard enough, whereas it's just biologically impossible for women to match the most athletic men. That's surely false, though: I'm sure plenty of men are also such that their genetic makeup precludes their ever becoming the world's top athlete. But perhaps the lack of a Y-chromosome is just especially easy to detect. As genetic testing becomes easier, can we expect to see more divisions to accommodate other unathletic genetic groups? What about unchosen environmental impacts, e.g. poor childhood nutrition?
So it seems it isn't fairness/handicap considerations that are in play here after all. Women are not uniquely disadvantaged, so maybe the thought is just that they are unique, simpliciter -- i.e. that men are women are different kinds of beings, in some deep metaphysical sense. An obese white man may be practically incapable of attaining the ideal form of a (male) sprinter, but it is a norm that applies to him nonetheless, in virtue of the kind of being that he is. There is no corresponding failure, it might be thought, on the part of a female Olympic athlete. She has achieved peak fitness as it applies to the kind of being that she is, namely a female. That some men are faster yet is no more relevant to assessing her than is the greater speed of a cheetah.
I suspect this is the sort of picture that underlies our common practices, though the metaphysics seems awfully dubious. Feminists, especially, will be rightly suspicious of essentializing sex differences in such a way. But it does seem to be a more attractive view of elite sport, at least: the point is not just to have equally handicapped people compete for competition's sake. Rather, the competitors should exemplify the peak of athletic excellence, as it applies to the kind(s) of beings that we are (humans, I should think!).
Then again, perhaps there is a more pragmatic story to be told. Elite sports serve an aspiration function, and regardless of the metaphysical facts it's surely true that sex/gender plays a large role in the subjective self-conception of many people. For pragmatic purposes, it's good to have sporting divisions to accommodate the types of beings that we take ourselves to be. So for this reason, it's good to accommodate female athletes, given our contingent cultural circumstances, even though in the ideal post-feminist world there would be no point since solidarity and group identification would not break down along sexed lines.
Any other suggestions?