Dear enlightened,
This is what I've heard about this.......
In Genesis 6: 1-2,4,9 Now it came to pass when men began to multiply on the face of the earth, and daughters were born to them,
that the sons of God (nephilium or demonic angels) saw the daughters of men, that they were beautiful: and they took wives for them selves of whom they chose.
vs 4 There were giants on the earth in those days, and also afterwards when the sons of God came into the daughters of men and they bore CHILDREN to them. These were the mighty men of old men of renown.
So here the bible is clearly telling of demonic biengs having relations with human women and children were born that were mighty men of old.
Also in Jude vs 6 it also talks about the angels who left left there proper domain and committed sexual immorality as sodom and Gomorah did.
So the theory is that angels came and what was born was half demon half human. Satan's purpose was to destroy God's promise that the seed of Eve would be born someday and crush his head. Satan wanted to destroy or impurify the seed of Eve so God would not bring the promised Messiah through a seed corupted like this.
Also in greek mythology (Which are ancient stories that could be based partially in fact) you have half human half demon or god like biengs. In the end these biengs all died in a flood.
In Genesis 6:9 This is the geneology of Noah. Noah was ajust man,.........perfect in his generations. And Noah walked with God.
In the Hebrew perfect in his generations is saying that Noah and his sons were not corrupted in there geneology meaning they had no demonic bieng mixed in there genes or DNA.
http://rr-bb.com/showthread.php?t=55927