"Many of the parents are high achievers and their children are also very bright too."
Inbreeding isn't about the stereotypical image of a cross eyed yokel sitting on a porch with webbed fingers and a huge adams apple while playing the banjo.
It's not about intelligence of the children but about their being an increased chance of regressive genetic conditions becoming increasingly common.
Cousins have an increased chance of this due to the likelihood of both of them carrying a regressive inherited gene that has been passed down from the common grandparent.
If cousins keen on marrying cousins then those regressive genes keep floating around rather than getting flushed out by outbreeding. The odds of the defect making an appearance therefore continue to increase.
This is a real and genuine risk that is not being exaggerated. Some simple statistics or a genetic family tree show the risk very clearly.
Should it be banned?
I'm very unsure, probably not, as people have said we don't ban all sorts of other things that lead to increased risks of genetic problems.
However people need to be educated about this and as long as there is an opinion that these risks are being exaggerated then that's clearly not happening.