All of biology is incomplete. Some of biology is more incomplete than other parts of it. While improving all the time it still seems the binary sex classification system is one of the more useful tools used. How does one defend Evolution from being considered wrong/incomplete without the use of it?
For humans, based on standard biology science currently, it is at least 99% accurate, is it not? And while the overlaps may vary from sport to sport, the distributions, and peaks, of athletic potential for those with XX chromosomes fall short of those for XY chromosomes, often markedly and most often very clearly.
For elite level competitive female sports the under 1%, the intersex, has long been problematic as to how to classify them and/or how to handicap or exclude them. But this is under 1%, which biology considers intersex. Presumably a similar under 1% in other mammals would also provide classification difficulties for biologists. (yet Biology doesn't consider interviewing any of the 99+ % to help in the classifications)
Further to, and mostly separate to that, drugs have been a problem in elite level sports, including deciding on fair use for medical reasons.
Despite those problems elite level competitive female sports has come a long way. Despite the problems the presence of intersex athletes presents, and despite the problems of drug use in sports.
Now many want to further include some non intersex athletes, potentially any of the biologically male half of the 99+%, to traditionally female only sports, both competitive and recreational. Some even claim this does not threaten female sports often citing the problems of including intersex athletes, as if they are the same thing, or as if they won't add to the problems otherwise already faced. Some claim the problem isn't sufficient to address ("just let them play"). Some believe that scientists or other experts can make it fair, even while arguing the limits of scientists on the subject.
If you want to include XY chromosome transgenders in female sports to support their acceptance in society what rules do you propose to use? Testosterone targets well above that of typical female range? Unhealthy targets within typical female range? Something else that you think both transgender and cisgender female competitors will be comfortable with.
If you want to include XY chromosome transgenders in female sports how do you define them? Do you simply let them define themselves? What questions do you ask that they ask of themselves? What experience do they fall back on other than their own, and only their own, while none have shared that experience? How do you ask them to differentiate themselves from, say, a gay male with XY chromosomes, with no known biological disadvantage, that also only has his own experience and judgement to fall back on?
One more bad analogy...😛...
...Ben...no let's call him Bert Johnson, identifies himself as a "Supermale" and has increased his testosterone levels under doctors orders for the sake of his overall health, both mental and physical or some combination there of. He was already a fast runner, and now runs even faster His Doctor thinks that's great, and like Bert also and wants him to compete at the Olympic level...who is anyone to deny him his place at the track? (his Doctor points out that his testosterone is merely twice normal levels, much less of an advantage than the 3+ times normal levels that current transgenders are allowed over typical females, and Bert retains no other advantages over his male counterparts)
Not that anything like that would every happen. (I have to add that as I know someone would be on me about it faster than anyone could say "East German Swimmer", or "why doesn't the Russian flag get played at the Olympics, or their anthem played when Russian athletes win?)