splendid "why do we emphasise sex?" I think out of politeness, although we may call a mixed group of children just children. But much like Ladies and Gentlemen.
Also because men boys/males tend to dominate and they are taken as the default.
How many times have I refereed to animals (which I do not know the sex of) or even inanimate object as Mr Hedghog or Mr Teddy. Or talked of unknown drivers in a car or people doing jobs as 'him' or 'he'.
Just think of just how many films, books, plays and stories have a male protagonist, whether they are written by a female or a male, Harry Potter, Charlie and the Chocolate factory, Toy Story, Shrek (true Fiona has a big part but it is still called Shrek) and countless others.
If we don't acknowledge girls and women we may forget they exist, since most of film, TV, books, history and media is driven and written and run by males.
cookiefiend are classes where you teach (if you do), or your kids go to school, divided on the grounds of sex? I know some whole schools are, and I can only think of a handful of groups like Guides that are single sex.
But in a mixed sex school classes are all mixed aren't they? With the exception of sex education or personal health or whatever it is called.
Which seems totally appropriate to segregate on the grounds of sex, since it would make sense to the kids to want to know primarily what was happening to their own bodies at puberty etc.