“...the sons of God saw the daughters of men, that they were beautiful;
and they took wives for themselves of all whom they chose" (Genesis 6.2).
Wild speculation abounds with respect to this verse, ranging from aliens impregnating women, to men of "noble" lineage mating with women from the lower classes.
But, by far, the most common view is this: the "sons of God" alludes to certain fallen angels who came to earth, joining in Satan's rebellion, mated with human women, and, consequently, bore a hybrid angel/human race called the 'Nephilim' — meaning, "giants, fallen, mighty ones." This view, however, is untenable.
First, Moses is careful to characterize the “sons of God” with human terminology. The subject of discussion concerns the ever-expanding population of the human race ["when men began to multiply"...v. 1]. The venue in which these events unfold occurs “on the face of the earth.” It is from this population and out of this backdrop that the “sons of God” emerge — hence, they were part of the expansion of humanity.
Equally so, after the “sons of God” married the “daughters of men,” the Lord became severely displeased with them [see below], causing him to “strive [contend] with man” (v. 3) — not with men and angels. These marriages became the catalyst for the global flood — a judgment rendered against “the wickedness of man” (v. 5).
Again, the Lord was sorry that he had “made man on the earth” (v. 6), and resolved to “destroy man...from the face of the earth.” As an aside, if the “sons of God” were insubordinate angels, how did this destruction serve as punishment for them? Did Satan and his minion drown?
More likely, then, the passage describes, from start to finish, the activities of the human race, and the sacred judgment, culminating in the global flood, which ensued as a result.
Second, it is true that angels, on various occasions in the past, manifested themselves in human form, and could be heard, seen, and felt by their human counterparts (cf. Gen. 3.1f, 2 Cor. 11.3, Rev. 20.2; Gen. 18.1f; 19.1-22; 32.1-2; Matt. 4.1f; Acts 10.4, 30).
However, it must be remembered that angels are, in their nature, “spirits” (Heb. 1.14), possessing neither flesh nor blood (cf. Lk. 24.39). As such, whether they seem to take earthly form, or retain their true spiritual form, they are, in their nature, incapable of fleshly procreation, as per God's designs for their race.
In contending with the Sadducees, Jesus affirmed that, in the resurrection, the human population shall no longer engage in procreation, since their nature will have been altered (some for glory, others for wrath, cf. Rom. 9.22-23). Instead, they will be “like angels in heaven” (Mk. 12.25).
Our Lord, through Matthew's account, alludes to the fact that angels specifically do not “marry” or engage in any sort of marital unions (Mt. 22.30; see also, Lk. 20.34-35).
All angels, regardless of their relationship with God, are inhabitants of this heavenly realm (cf. Mt. 24.36; Eph. 6.12), and, as such, do not actually consist of “flesh and blood” (1 Cor. 15.50). Only those who are actually born of flesh assume the fleshly components and characteristics of their progenitors — spirit beings remain spiritual in nature, in spite of their ability to alter their appearance (cf. Jn. 3.6; 2 Cor. 11.14).
Does the fact that Satan is able to appear as an angel of light mean that he actually becomes an angel of light? Certainly not (2 Cor. 11.13, 15). The appearance and the substance are two different things. Neither can angels, who are spiritual beings, actually become flesh and blood, by their altered appearance.
In light of this information, the “sons of God” of Genesis 6.2 likely does not allude to angelic beings, since (1) they are spirit in nature, and (2) they were not designed to be propagating creatures.
Third, some suggest that the "giants" of those days alludes to the offspring of those angelic/human relationship. But the 'Nephilim' or "giants" existed both before and after the "sons of God" took wives of "the daughters of men," and thus were not the direct descendants of those unions (Genesis 4.4 - "there were [already] giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men..."). The "sons of God" and the "daughters of men" were not the original progenitors of the "giants."
Fourth, the 'Nephilim' are referenced again in Numbers 13.33. As the Israelites, under the leadership of Moses, spied out the land of Canaan, they found the land to be fruitful, but they regarded the people of the land as impediments. They said:
"There we saw the giants [Nephilim] (the descendants of Anak came from the giants); and we were like grasshoppers in our own sight, and so we were in their sight."
Now observe this: Genesis 6.2 occurs before the global flood. If there were a mixed-race of angelic/human beings, they did not survive the flood, since Noah and his family were the only people to be saved from that destruction (cf. 1 Peter 3.20-21). But Numbers 13 takes place after the flood. Since the "giants" still exist after the flood, this would indicate that the giants were human in nature, descending from the bloodline of Noah, like the rest of us. Thus, the giants could not have been a mixed angelic/human race.
Fifth, Genesis 6.4 describes these “giants” as “mighty men who were of old, men of renown.” Too, Numbers 13.32 specifically characterizes these "Nephilim" as "men of great stature." Thus, they were tall (the word Anakim suggests, 'long-neck'); and, likely, strong and formidable. But they were men nonetheless — not the offspring of angels.
Likely, the meaning of Genesis 6.1-4 is this: the "sons of God" alludes to righteous men — men who, like Seth, "called on the name of the Lord" (Genesis 4.26). The "daughters of men" would thus be a reference to unrighteous women — who did not "walk with God."
The Scriptures frequently describe righteous men as God's "children" or "sons" (cf. Deuteronomy 14.1; 32.5; Galatians 4.7; Hebrews 12.7).
The gist of the text, then, is as follows: as the human population began to expand (6.1), the righteous men (or "sons of God") began to mix with unrighteous women, owing to their beauty, and bore children through them (6.2). When righteous men began to devolve their spiritual pursuits for fleshly pleasures, and when the “mighty men of renown” intensified their carnality, the milieu of the world descended deep into the abyss of sin, so that succeeding generations began to “strive with God.”
Spiritual training having been all but vanquished (with Noah alone receiving heaven's approbation, v. 8), the wickedness of man became “great in the earth” so that “every intent of the thoughts of his heart was only evil continually” (Genesis 6.5).
To remind his creation of his awesome might and indelible justice, the Lord generated a world-wide flood, wiping out nearly the entire human population.
And to remind his creation of his awesome grace and indelible long-suffering, the Lord spared Noah and his household, preserving human life in order to offer redemption through Jesus Christ.
The sons of God: righteous men.
The daughters of men: unrighteous women
Their children: did not know God, since their righteous fathers abandoned their righteousness, when the beauty of the flesh won them over.
The Nephilim: giants. Tall and mighty men (proud in their ways), but not necessarily descendants of these unions. Genesis 6.4 places the existence of the Nephilim and the inter-faith marriages around the same general period, but not necessarily as one resulting from the other.