Genesis 6:1-4

When humankind began to multiply on the face of the earth, and daughters were born to them, the sons of God saw that the daughters of humankind were beautiful. Thus they took wives for themselves from any they chose. So the LORD said, “My spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years.” The Nephilim were on the earth in those days (and also after this) when the sons of God were having sexual relations with the daughters of humankind, who gave birth to their children. They were the mighty heroes of old, the famous men.