Jonah, there is a greater likelihood of birth defects in cases of cousin marriage. That is an argument against the practice, but it may not be a decisive one, because the effect is not massive. Advocates of cousin marriage in Britain note that the likelihood of birth defects is not significantly greater than the increased likelihood of trouble in births to women over 40. And so they say, we wouldn’t ban women over 40 from marrying, so why ban cousin marriage?
For me, the decisive issue is the social-cultural effect of the practice. The genetic problem is an important additional issue, but by itself it is not the main issue, at least as I see it. But the details on all this are in some dispute. I will probably have more to say about the issue down the road.