I always viewed it as wrong (despite family beliefs) until I read The Unquiet Earth, which created a whole new perspective. I did not know that it was not considered wrong on other areas of the world.
My only knowledge of it was a family that lived down the street from me while I was growing up. They had 5 kids. 3 were blind, 4 were deaf, and they all had learning problems. It was known that they were first cousins and everyone said that was why. However, with this new information (1 in 1000 marriages in the US), I really have to wonder.
I work with a woman who moved to the US to marry her uncle and their relationship seems very natural, but they never had children and the thought does seem a bit weird to me...but maybe I am a bit of a prude?