Are you purposely being obtuse?
Of course, with their physical differences, boys on average are better than girls in most sports, and in contact sports mixing much larger boys with the frailest of the girls would be dangerous.
So if most girls are going to have opportunities to play sports, as many would like, they need separate teams.
Yet, there are some girls who are exceptions and can play at the boys’ levels in particular sports. If there doesn’t happen to already be an equivalent girls team for such girls to play on (or even if they’re just that much better so as to be competitive with the boys), what is the problem with letting them play on the boys team?
Okay, I see your answer above re: thinking it’ll somehow train boys to be aggressive to girls outside of the game situation.
I don’t really see that as a/the problem, but fair enough. Except only a couple of sports have that kind of direct physical aggression. E.g., wouldn’t matter a bit if she were a pitcher in baseball, unless you are against all sorts of coed sports—which I think is crazy.
It is more respectful social interaction—e.g., playing tennis, volleyball, or golf together—rather than less that better socializes men to women, I’m quite certain.
Believe it or not, some boys have an ethical issue with treating a girl with the same level of force that they do other boys. They would rather not try, than risk breaking her cheek bone with an elbow. They prefer not to wrestle a girl using positions and holds suited to a rapist. They would rather not pitch inside to back her away from the plate.
To accommodate the admittedly out of place girl (sorry, not everyone has every opportunity in life), you neuter boys' sports.
Girls are not entitled to be boys. If there aren't enough girls to form a team, they have no more right to play than if a boy wants to play a sport that isn't offered.
I've coached co-ed soccer up to age 14, and after seeing girls get broken bones simply from getting hit by the ball, I'm certain that co-ed should stop at 12.
Acknowledging reality has never been defined as being obtuse.