It seems rather odd to me that it is okay to separate boys and girls by a wall or put them in separate schools but it is not okay to put them on separate sides of the class. The former seems a more extreme separation to me than the latter.
It depends on the reasons for the separation I guess. Historically in the UK I would imagine that girls and boys were separated for reasons of propriety, because it was deemed somehow 'improper' that they should mix in case they were 'tempted'.
For sexual equality reasons, we have moved on from this now, so that we learn from a young age to deal with being in an environment with the opposite sex, since that after all is the real world - we aren't in Jane Austen times any more.
Yes, girls' and boys' schools still persist, as an anachronism more than anything else. I think the 'distraction' argument is the main justification these days, but single sex schools do seem outdated now, really.