For some informed data on the topic instead, try http://www.nytimes.com/2002/04/04/us/few-risks-seen-to-the-c...

or, more recently: http://www.nytimes.com/2009/11/26/garden/26cousins.html

"Dr. Motulsky said researchers did not know why marriage between cousins was viewed with such distaste in the United States. He said some of the revulsion might have stemmed from the eugenics movement, which intended to improve the human race by deciding who should be allowed to breed. The movement flourished in this country early in the 20th century."

Anti-cousin-marriage is a very recent and very American thing, and stems from the same movement that gave us nazi racism.