I was reading a Harry Potter fan-fic the other day and the author couched the notion of Pureblood Supremacy in a way I hadn't considered before.
Looking at Magical beings as a step above Muggles in the evolutionary ladder, I don't know why anyone would be surprised that some within the magical community wouldn't imagine that they were better. And are they really wrong?
I recall one story where the rep from Hogwarts was talking with the Granger parents and explained that by learning magic Hermione would never again have to worry about trivial things like being warm, having money, eating well, getting cavities, etc. A creature capable of harnessing magic really did get a step up in life in very measurable ways.
How then could they not think they were superior to Muggles? More importantly, how could they not worry about interbreeding with Muggles causing problems within the magical community? Seems to me that diluting your gene pool sounds good in terms of interbreeding too much, but also a bad thing in terms of diluting or killing off your ability to harness magic. With witches and wizards populating every continent on earth, inbreeding would be a very tiny worry since it would be so easy to meet other beings like yourself.
That's not to say the baddies in that story were justified in their treatment of Muggles, and I think this is another example of JKR's horrible world-building skills.
If it were me, I'd have a hard time justifying anyone magical from even thinking about marrying a muggle and thereby almost certainly stripping their children of any chance at being magical. Even a Squib would, I think, fight hard to marry into a magical family in the hopes that whatever was wrong with them wouldn't be passed on to the children.
Also, I don't see the Magical community being hateful towards Muggleborn witches and wizards any more than they'd be hateful towards witches and wizards born to Italy or France. Regardless of their circumstances, they are proven magical beings and part of the overall community. At least, that's how I see it if we're trying to be realistic about things. You could hate a muggleborn because they aren't part of the aristocratic class that's been around a thousand years, sure, but not because they happened to be "muggles with magic" because they aren't really muggles once they've demonstrated the ability.
One of these days, I'm going to do a proper breakdown of the Universe to get all the little details squared away.