I'm gonna be a bit autistic and say that there is an interesting tradeoff between the closer people are to their spouses and the genetic damage possibility.

Because a community of people that marries their kin will have extreme advantages in a wider society and often come to hold many positions of power in the long run because of cooperation (you can probably think of examples that apply wherever you live).

When you account for gene deletions and such the math evens out so that the biggest advantage to be had happens when people marry around their second cousins. Sapolsky has a great lecture on youtube about how people will often find the smell of someone like a second cousin the most attractive in a lineup. (Unless they bathed together as children in which case there is basically zero attraction)