The result was that in any one generation there were but a few hundred of them, all closely related. They kept this up for more than half a millenium. Even today one is King of Spain. Still within the family, but not named Bourbon, we find the Princess Bouval-Craon who runs Sotheby's Paris. I have no idea who she married, but any of the identifiable members of this narrow group probably continue to chose spouses from among the others.
When the poor white trash in the Appalachians do it, it's suddenly a crime against nature.