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.