What's wrong with choosing a different partner to reproduce with and having a different partner raise children with?

Ignore the fact that people call it "cheating".

Say there's 1 person who would make a great father because of his nurturing nature and ability to provide for a family, but genetically he isn't a good candidate to pass the genes on for (short, ugly, not muscular). Another person has great genes but lacks fatherly characteristics (unreliable and financially unstable).

What is the harm in marrying the first person but passing the genes of the second person? For the greater good of the human race it seems better.

What's wrong with choosing a different partner to reproduce with and having a different partner raise children with?
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