Polygamy Returns to Modern Cultures

It looks as if a form of polygamy is returning to modern human culture.  

Seven percent of adult males in the US are sterile.

If reproduction of humans is trending toward using sperm banks and if people pick from
sperm banks as has been reported, then some men’s sperm is much more popular than other
men’s sperm and the tendency is for some men to have many offspring and other men to have
few.  King David in the Bible had many wives and many children.  A sperm bank donor who
seems by description to be intelligent, athletic and well rounded (a lawyer, Harvard graduate,
and former Olympic swimmer) will ending up having many, potentially thousands of children,
by various “wives” who obtain his sperm via the sperm bank.  King David would have been
responsible for supporting his children but this modern man is not.  It would not be polite to
call the men who act as fathers to other men's children using the name "eunuch," but that is
the role that he is playing to some extent.

Question: How does this trend play out?  How do the offspring of one man avoid marrying their
brothers and sisters?

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