I can't see where people see this as a matter of principle. I went to a single sex school, my wife went to a single sex school, our son went to a mixed school. There are advantages and disadvantages both ways. One size does not fit all.

The analogy with separate schools for black and white pupils just doesn't stand up, so far as I can see. For one thing most black and white children aren't brothers and sisters living in the same houses, with the same parents and the same family income between them.

I suspect that a mixed system might possibly on balance be a little bit better for the education of boys, and single sex system better on balance for girls. I don't know if any research has been done to determine whether that is true.

As for uniforms, the impression I get is that if a uniform isn't imposed by the school authorities, a uniform gets invented by the children, and enforced by what is rather euphemistically called "peer pressure". Either way kids who don't conform tend to have a rough time.