I believe the problem is never showing evidence, but that the evidence is overwhelming. I could explain the general idea and, maybe, one or two specifics. People that use the XX/XY binary argument wouldn’t be able to explain either, but it’s usually only used because it conforms to a bias. And we are only talking about humans here. Language would implode if we tried to maintain convenient binaries and still back it up with science.
We draw the line by weight classes and/or muscle mass, not by what’s in person’s pants. It doesn’t have to be just 2 categories or the same categories for every sport, either.