Gender Segregation

I've always wondered why advocates of gender equity condone segregation in sport.
Surely open competition is fairer.

Well, Jumpy, I'm quite happy to have sports segregated into classes based on mass/weight to even up the game. For instance, Boxing (which I don't like) shouldn't have flyweights up against heavyweights - that would be unfair.

However, what the blazes would be wrong with having mixed teams in Cricket or Soccer/Football? Mixed doubles works well in Tennis, so what's the difference?

Away from sport: what's wrong with having boys and girls in separate classrooms for one or two lessons in each school day? It's not going to cause the Collapse of Civilization, is it?