For I am come to set a man at variance against his father, and the daughter against her mother, and the daughter in law against her mother in law. And a man's foes shall be they of his own household. Matthew 10:35
The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. -- Genesis 6:2
There seems to be no good answer to this.
The traditional Christian view is that the "sons of God" were angels, and that these angels came down from heaven and had sex with women.
But the Judaism seems to think they were sons of Seth.
I'll let Wikipedia try to sort if all out for you.