What percentage of a person's ancestors, going back 100,000 years (when people lived in kin groups), mated with their cousins, siblings, or other relatives? Perhaps nonrelative mating, like nuclear families, is a relatively recent social invention. Cousin marriage has the added binding agent of blood relation.
All kidding aside, even just the past two hundred years in some parts of the country (not talking about deep south either) marriages between cousins was common - there wasn't exactly a large pool of potential mates to choose from in many small towns until this past century. I'm sure everybody has had a few kissin' cousins in their past.