The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
Genesis 6:4 (NIV)
The actions of the "sons of God" here are written in stark and glaring contrast to Genesis 4:1, where it says that Adam knew his wife Eve.
I believe the text is describing violence.