It's not marrying your cousin per say, it's having children from a small gene pool.
It's happening in Amish communities too.
Historically Martha's Vinyard had a very large hereditary deaf population, to the extent that everyone on the island used sign language - not ASL but their own language.
It was used by deaf and hearing people, there is an account of women on neighboring farms going out with a telescope each, one would start looking through the telescope while the other would sign and then the process was reversed. Very useful before the telephone.
I think when you are part of the majority it is difficult to see how ingrained cultural beliefs are about what is normal.
When I was teaching a BTEC unit 'equality and diversity' I would often ask the class to put their hands up if they washed meat before cooking. It always split the group and often there was some robust debate.
I've also seen a similar debate happen on here.
I've been told it's OK for diabetics to eat anything grown above ground but not things grown below ground. This is a cultural belief.
Now there is some truth in this when it comes to vegetables with some exceptions, sweet potato is one, but fruit is a different matter.
I think there is a lack of cultural understanding in healthcare and research and as I said before the shadow of eugenics makes it harder to say' "I'm going to research X group".