See especially Genesis 6:5 to start... "And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually."
Genesis 6:1 - 22
ESV - 1 When man began to multiply on the face of the land and daughters were born to them. 2 The sons of God saw that the daughters of man were attractive. And they took as their wives any they chose.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.