Then the people began to multiply on the earth, and daughters were born to them.
The sons of God saw the beautiful women and took any they wanted as their wives.
References for Genesis 6:2
Then the LORD said, “My Spirit will not put up with humans for such a long time, for they are only mortal flesh. In the future, their normal lifespan will be no more than 120 years.”
References for Genesis 6:3
In those days, and for some time after, giant Nephilites lived on the earth, for whenever the sons of God had intercourse with women, they gave birth to children who became the heroes and famous warriors of ancient times.
The LORD observed the extent of human wickedness on the earth, and he saw that everything they thought or imagined was consistently and totally evil.
So the LORD was sorry he had ever made them and put them on the earth. It broke his heart.