Ooooops, yeah, major reading comprehension fail on my part. When I read that chapter, I just kinda sped past the squee-ing girls to get to the story, and ended up still seeing things through the lens of canon and Harry's previous impression of "Damn, these Wizards totally missed out on the Enlightenment!" Guess I need to pay more attention to preconceived notions and not letting them cloud my vision. :)

With more reflection though, it does make sense to me that Wizards would have a more enlightened attitude toward LGBTQ people, and find other irrational reasons to hate each other. In a world where some people can turn into animals, or alter their bodies at will (Metamorphomagi), and anybody with a jug of Polyjuice Potion and a clipping of hair can change their physical sex, non-heteronormative sexual identity could be seen as pretty tame. McGonagall could, if she so desired, turn into her feline form and go out lookin' for some tom. Or if she's lesbian or bi, then McGonagall/Mrs. Norris.

So, yeah.