Here is some info I think I pasted before on this. But I too find it an interesting thing to think about and how bad those dudes must have been. Freaks to say the least...
Who were the ‘sons of God’ that fathered the Nephilim?
Who were “the sons of the true God” that were involved? Were they men who were worshipers of Jehovah (as distinguished from the general run of wicked mankind), as some claim? Evidently not. The Bible implies that their marriage to the daughters of men resulted in whipping up the badness in the earth. Noah and his three sons, along with their wives, were the only ones in God’s favor and were the only ones preserved through the Deluge.—Ge 6:9; 8:15, 16; 1Pe 3:20.
Hence, if these “sons of the true God” were merely men, the question arises, Why were their offspring “men of fame” more than those of the wicked, or of faithful Noah? Also, the question might be asked, Why mention their marriage to the daughters of men as something special? Marriage and childbearing had been taking place for more than 1,500 years.
The sons of God mentioned at Genesis 6:2, therefore, must have been angels, spirit “sons of God.” This expression is applied to angels at Job 1:6; 38:7. This view is supported by Peter, who speaks of “the spirits in prison, who had once been disobedient when the patience of God was waiting in Noah’s days.” (1Pe 3:19, 20) Also Jude writes of “the angels that did not keep their original position but forsook their own proper dwelling place.” (Jude 6) Angels had the power to materialize in human form, and some angels did so to bring messages from God. (Ge 18:1, 2, 8, 20-22; 19:1-11; Jos 5:13-15) But heaven is the proper abode of spirit persons, and the angels there have positions of service under Jehovah. (Da 7:9, 10) To leave this abode to dwell on earth and to forsake their assigned service to have fleshly relations would be rebellion against God’s laws, and perversion.
The Bible states that the disobedient angels are now “spirits in prison,” having been ‘thrown into Tartarus’ and “reserved with eternal bonds under dense darkness for the judgment of the great day.” This seems to indicate that they are greatly restricted, unable again to materialize as they did prior to the Flood.—1Pe 3:19; 2Pe 2:4; Jude 6.
Insight Book #2.