Some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction. 2 Peter 3:16
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 these angels came down from heaven and had sex with women.
But there are many other interpretations.
I'll let Wikipedia sort if all out for you.
