“And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.” (Gen 6:1-5)
The Angelic Groom
Referencing the Enochic understanding within Jude 1:6-7 and 2 Pet 2:4, St. Irenaeus accepts the idea that two hundred fallen angels (or “Watchers”) were responsible for introducing much wickedness into the human race, teaching humanity witchcraft and every kind of vanity. The offspring of this union are the giants known as the Nephilim.
“For unlawful unions occurred on earth, as angels united themselves with daughters of men, who bore them sons who, because of their exaggerated height, were called giants. The angels then gave their wives, as gifts, wicked teachings, for they taught them the powers of roots and herbs, of dyeing and cosmetics, and the discovery of precious material, love-potions, hatreds, loves, infatuations, seductions, bonds of witchcraft, and all kinds of divination and idolatry hateful to God. When these entered the world, the things of wickedness over-abounded, while those of righteousness decreased, until judgement came upon the world from God…” –St. Irenaeus of Lyons
The angelic perspective is the majority view of the Church Fathers, and it is found in people like Justin Martyr, Athenagoras, Origen, Clement of Alexandria,Hilary of Poitiers, Tertullian, Eusebius, Ambrose of Milan, Jerome, and Methodius.
However, others like Ephrem the Syrian, John Cassian, John Chrysostom,and Augustine instead taught that the sons of God refer to the lineage of Seth, because angels are incorporeal beings and cannot have bodily relations with humans.
The Conduit Bride
This brings up a dilemma within certain interpretations of the text. Both perspectives raise valid points: Fallen angels had to be part of it, but how can a person reconcile the idea that angels are incorporeal in relation to man, yet affirm the traditional angelic interpretation of the text? And if angels really did mate with human women, and if this was truly a major reason why God sent the flood, why do the Nephilim (such as Goliath) still exist even after the flood? The only possibility is that the fallen angels united with women again after the flood. But how? I want to offer an alternative perspective that synthesizes the best of both worlds: I believe it is possible that women offered themselves as sexual conduits for demonic spirits through occult ritual.
Angels are incorporeal, so they cannot literally have physical relations with humans. However, they can certainly unite spiritually with a woman by possessing her, and such a union is so deep it could be considered a kind of marriage (essentially the perverse opposite of a nun being ‘married’ to Christ). Fallen angels can reveal to her the mysteries of the occult, and demonic possession could also have an effect on pregnancy. Scripture testifies of a demoniac with super-human strength (Mar 5:3-4), so it is not a stretch to think someone like Goliath is the result of such a union. This also explains why the Nephilim continued to exist after the flood, since temple prostitution with oracles was commonplace. There may have even been many more giants in the world if such practices did not go hand in hand with child sacrifice.
Therefore, I think this interpretation is interesting and does a good job when it comes to answering the difficulties surrounding the incorporeal nature of angels, but without doing any kind of harm to the other interpretations.
 1 Enoch 6-9.
 Demonstration of the Apostolic Preaching, 18, 19.
 1 Apology 5.
 Plea for the Christians, 24.
 Against Celsus, LV.
 Miscellanies, 5.1.10
 Commentary on Psalm 133:3.
 Idolatry 9; Veiling 7.
 Preparation, 5:5.
 Noah and the Ark 4.8.
 Hebrew, 6.4.
 Discourse on the Resurrection, III.1.7.
 Commentary on Genesis 6.3.1.
 Conferences, 8.21.
 Homily on Genesis, 22.6-8.
 City of God, 15:22-23.