Are these "sons of God" angels, as some have concluded? Can angels reproduce?
Answer:
Scholars debate and disagree over the meaning of the obscure reference to "the sons of God" in Genesis 6:1-4. Some people read into these verses the idea that it refers to angelic beings marrying women and producing a race of giants. Christ explained that is impossible, teaching that angels are neither male nor female (Luke 20:34-36)—that is, they are incapable of reproduction.
Humans are clearly the subject in Genesis 6—not angels. God said, "My Spirit shall not strive with man forever, for he is indeed flesh" (verse 3, emphasis added throughout) and, "I will destroy man whom I have created from the face of the earth" (verse 7). If angels were capable of reproducing as mankind does, their offspring would be spirit, according to the principle of "according to its kind" portrayed in Genesis 1.
The Bible labels the offspring of these marriages "giants" (Genesis 6:4), meaning simply people of giant stature. Similar people are spoken of in later times, most notably Goliath and his family.
How, then, can we understand Genesis 6:1-4? Human beings are also sons of God. We're not referring to becoming spiritual sons of God through conversion, but to the fact that all people are sons of God by creation (Luke 3:38). The attitudes and actions of these "sons of God" were so wrong that they provoked God to send the Flood.
Halley's Bible Handbook raises the possibility that these sons of God were the descendants of Seth. Seth, the Bible records, was made in the image of Adam, who was made in the image of God (Genesis 1:26; 5:1-3). Speaking of Seth's descendants, Genesis 4:26 adds, "Then men began to call on the name of the Lord," a phrase that could also be rendered, "called after the name of the Lord"—that is, the "sons of God." If so, the women, "the daughters of men" whom these "sons of God" married were the descendants of unrighteous Cain. By marrying these women, the sons of righteous Seth turned from God, leading Him to say that the entire world was then corrupt (Genesis 6:5-7, 12).
An alternative explanation is that "sons of God" in Genesis 6:2 refers to self-willed men who called themselves "sons of god," not in worship of the Creator, but of pagan deities. Their marriages were in defiance of the Creator God, as they lived contrary to His will. In light of God's characterization of society riddled with violence (verses 11 and 13), we surmise that the men forcibly took the women as wives.
Regardless of which explanation is accurate, the idea that a half-spirit, half-human race resulted from angels marrying women is impossible, according to the Bible.
Mt. 22:30 describes how "angels of God in heaven" behave, not how fallen angels would behave. The phrase "Sons of God is a very old phrase and only appears in the two oldest books in the Bible, Gen & Job. Clearly in Job 38:7 "Sons of God" refers to angels so if for no other reason than to be consistant in translation it warrants a deeper look. This period of time on earth was preflood and quite early in demons experiance with man. Mankind must have presented quite a tempting toy for a group of beings banished from heaven by the very Being these weak creatures were fashioned after. Manipulating them and possessing them was likely quite a sport for them. We see many examples of demon influence and possession of men and beasts throughout the Bible. While I doubt that a cross species mating occured for all the complications it would present later, it's not impossible to conceive of the idea that fallen angels considering men as pets to be managed, looked at the giants which, as you point out, already existed and sought out men and woman who carried the Giant Gene and possessed them for the purpose of spawning more giants. They obviously accelerated the corruption of man until "the thoughts of his heart was only evil continuously." How much pleasure they must have had at the expense of these little God lookalikes! This theory while only a theory, not only allows you to more consistantly and less subjectively explain a rare phrase of the OT, it also helps explain two difficult NT passages.
1Peter 3:18-20 describes Christ apparently correcting misbehaving spirits during this exact timeframe who after being disobedient in some way were being restrained while Noah built the ark. If these spirits had done as I proposed earlier, they would certainly have earned a preaching to and maybe even a stern warning complete with a sign as a reminder to them as to who the woman was created for, which brings me to the 2nd difficult scripture my theory clears up;1Cor 11:9-10. Here a peculiar addition to the end of vs 10 perplexes all who read it,"because of the angels" Paul states the man was not made for the woman but the woman for the man and then says that a womans long hair is a sign or symbol of this,"because of the angels"! Just like the rainbow was a sign that God would never destroy man with a flood again, might woman's hair be a sign to the angels not to touch the woman!