My understanding is that 'Pure Blood' familes are usually the oldest wizarding families with histories that stretch back to the era when Witch burning and Witch hunts were common place. Much like American wizards there is the belief that personal contact with Muggles in anyway risks exposure of the wizarding world and could lead to war. Thus anyone who goes to such lengths as to have children with a Muggle is committing Treason agaisnt the Wizarding world.

Anti-Muggle(born) ideals used to be about protection, but in the time since witch hunting, they turned into a sense of superiority. At the time they separated from Muggle Society, wizards had better education, medicine, transportation. As burnings became less of a threat, they stayed separated not only to try to keep themselves secret and safe, but because most of them genuinely didn't think that the Muggles had anything to offer them. That's why they still use quills, parchment rolls, and robes. They think they've reached the top and refuse to change. That's also why the 'purest' families tend to also be the most old fashioned.