Ok, so you know that really creepy verse in Genesis about the Nephelim ? Well in case you didn't it is in Genesis 6:4-6 "The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown. Then the Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually. And the Lord was sorry He had made man on the earth, and He was grieved in His heart."
If you are anything like me, you are probably thinking to yourself, Oh my gosh some studly angel came down from heaven and seduced some 16 year old girl & BAM ! out came Hercules. Even though understanding every minor thing about Genesis is a) impossible b) not relevant to our relationship with God, it is still fun to speculate. The important thing to recognize is that God is not concerned with the how but the when. If He does not clarify, He obviously doesn't think it important for us to understand.
3 theories for sons of God :
1) Sons of God actually refers to sons of the line of Seth. Nephelim would refer to the sons of Seth (through whom the line continues) intermixing with the daughters of Cain (who was cursed) creating offspring unsightly to God.
2) Sons of God = angels. In other parts of the Bible (especially in Job) sons of God refers to angels. A man both human & angel would be considered an abomination. This could be another reason for God wanting to wipe out the Earth through the flood, to rid the Earth of such people.
note: a fault with this theory is that angels do not marry ? Do they have specific genders?
3) The third theory is more recent. Actually the first time i heard of it was when i was in Israel and our tour guide ( a seminary professor who majored in O.T. scripture, especially Genesis) stated that he believed sons of God referred to kings of old. Kings in that time were considered pure and were not supposed to mar there purity through unclean women.