SInce "sons of God" (as has been pointed out) can refer to either fleshy offspring or spiritual offspring, why not take Jesus literally when he explained to one group "Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning..."
This John 8 reference contrasts with Paul's observation that "they which are of faith, they are the children of Abraham." Is faith inherited? Did Timothy literally inherit his mother's and grandmother's faith, as Paul suggested? Does the seed of an unbelieving husband become sanctified through the believing wife, "lest your children are unclean; but now are they holy"? Are there two distinct spiritual lines of human descendants--one proceeding from Cain (the sons of men, spiritually the sons of Satan), and one proceeding from Seth (the sons of God)?
That's a less outlandish conclusion than the notion that "sons of God" and "daughters of men" in Genesis 6 describe naughty angels in flagrante delicto with easy earth girls. That concept flies in the face of reason if you accept what Jesus and others said about the nature of angels:
1. He makes his angels spirits, his ministers a flame of fire (Psalms 104, Hebrews 1)
2. A spirit has not flesh and bones, as you see I have (the resurrected Jesus, Luke 24)
3. For in the resurrection they [resurrected humans] neither marry, nor are given in marriage, but are as the angels of God in heaven. (Matthew 22)
Everyone imagines that angels have a corporeal dimension, but they apparently do not. A spirit does not have flesh and blood, Jesus stated. This makes angels incapable of sex, which Jesus confirmed when he talked about their celibate state in heaven: they don't marry because they are not made of flesh and blood. No need to procreate! God made plenty of them to begin with, knowing in advance how many he would need to accomplish his purposes.
I suppose fallen angels' innate incapacity to jump in the sack is why they so eagerly inspire sexual misconduct among humans; they can only enjoy it vicariously, and are content just going along for the ride. All that gothic stuff about succubi and incubi is just wishful thinking, in my opinion.
If Abraham sat and ate with angels, that's no proof of anything. We have no verifiable, empirical data with which to intelligently talk about "spirits" at all. Ectoplasm? Who knows, it's all hollow conjecture once we move beyond the revelations of scripture. From a religious standpoint, if we accept the Bible account as true, we can only admit that spirits (though non-corporeal) have been known at times to appear as humans, and at least on one occasion, to enjoy a good meal.
If a spirit has no flesh or bones, I doubt the food Sarah prepared for her celestial visitors traveled very far after it was swallowed.