When the number of people started to increase throughout the fertile land, daughters were born to them.
References for Genesis 6:1
The divine beings saw how beautiful these human women were, so they married the ones they chose.
The LORD said, "My breath will not remain in humans forever, because they are flesh. They will live one hundred twenty years."
References for Genesis 6:3
In those days, giants lived on the earth and also afterward, when divine beings and human daughters had sexual relations and gave birth to children. These were the ancient heroes, famous men.
References for Genesis 6:4