The story of Noah begins:
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When mankind began to multiply on the earth and daughters were born to them, the sons of God saw that the daughters of mankind were beautiful, and they took any they chose as
wives for themselves. And the Lord said, “My Spirit will not remain with mankind forever, because they are
corrupt. Their days will be 120 years.”
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Genesis 6:1-3
But the words I bolded have a different meaning in Hebrew. Instead of "wives" it is actually "women", and instead of "corrupt" it is actually "flesh". So now we have:
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When mankind began to multiply on the earth and daughters were born to them, the sons of God saw that the daughters of mankind were beautiful, and they took any they chose as
women for themselves. And Yehovah said, “My Spirit will not remain with mankind forever, because they are
flesh. Their days will be 120 years.”
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Now the cause of Noah's flood becomes clear. Mankind had become promiscuous, so it had to be wiped out.