Everyone who marries marries their cousin, it's just a matter of degree. Before the advent of the automobile, people traveled a lot less. Even more so as you go further back. Combine that with families having a lot more kids (you might have 36-64 surviving first cousins), and you've got a situation where nearly everyone you interact with might well be only a couple degrees of separation by blood. Marriage between first cousins has historically been a bit taboo, but so called third and fourth degree (aunts and uncles, first cousins) marriages were still pretty common. It wasn't really until the rise of the eugenics movement that the modern taboos and legal prohibitions were established.

I've been doing a fair bit of genealogy lately, and you can see on the family tree pretty clearly when people moved from from smaller, insulated communities to larger cities. Above that point, the tree fans out a lot less.