In the Bible mankind (humans) are referred to as "Son's of man" (being born from adam)
Son's of God refers to angels. In this case it is speaking about the fallen angels (the devil and his evil co-hort).
Huh? How can the fallen angels who are pure spirit beings have conjugal relations with humans and beget offspring?
THE NEW AMERICAN BIBLE
1 [1-4] This is apparently a fragment of an old legend that had borrowed much from ancient mythology. The sacred author incorporates it here, not only in order to account for the prehistoric giants of Palestine, whom the Israelites called the Nephilim, but also to introduce the story of the flood with a moral orientation--the constantly increasing wickedness of mankind.
2 [2] The sons of heaven: literally "the sons of the gods" or "the sons of God," i.e., the celestial beings of mythology.
3 [3] My spirit: the breath of life referred to in Genesis 2:7. His days . . . years: probably the time God would still let men live on earth before destroying them with the flood, rather than the maximum span of life God would allot to individual men in the future.
4 [4] As well as later: According to Numbers 13:33, when the Israelites invaded Palestine and found there the tall aboriginal Anakim, they likened them to the Nephilim; cf Deut 2:10-11. Perhaps the huge megalithic structures in Palestine were thought to have been built by a race of giants, whose superhuman strength was attributed to semi-divine origin. The heroes of old: the legendary worthies of ancient mythology.
JERUSALEM BIBLE
An obscure passage (from the 'Yahwistic' tradition). The author uses a popular story of a race of giants, in Hebrew Nephillim, the Titans of eastern legend, born of the union between gods and mortals. The author does not present this episode as a myth nor, on the other hand, does he deliver judgement on its actual occurrence; he records the anecdote of a race of supermen simply to serve as an example of the increasing human malice that is to provoke the Deluge. Later Judaism and almost all the earliest ecclesiastical writers identify the 'sons of God' with the fallen angels; but from the 4th century onwards, as the idea of angelic natures becomes less material, the Fathers commonly take the 'sons of God' to be Seth's descendants and the 'daughters of men' those of Cain.
The Lesson is taken from the book concerning Noah and the Ark by St. Ambrose the Bishop
We read that the Lord was angry. However, God did bear in mind (that is, he knew) that man was hard put to remain sinless, placed, as he was, in flesh on earth ; for earth is the home of temptations, and the flesh is the enticement of corruption. Yet man had a reasonable soul, and his soul had power to control his body ; and, being so made, he nevertheless struggled but little to keep himself from falling into that from whence he would not desire to return. God's thoughts are not as man's thoughts ; in him there is no such thing as change of mind, no such thing as to be angry and then cool down again. Therefore, anything said concerning God's anger is to be understood as setting forth the grievous nature of our sins, whereby we have (so to speak) merited the divine wrath. For iniquity had grown to such a degree that God, who by his nature cannot be moved by anger, or hatred, or any passion whatsoever, is represented as provoked to anger.
And God threatened that he would destroy man. He said : I will destroy man, whom I have created, from the face of the earth ; both man and beast, and the creeping things, and the fowls of the air. Wherein had the brute beasts offended? They had been created for man's use ; and, when man was destroyed it followed that they must share the same fate because they were no longer of any use. And there is a higher reason : Man is a living soul, capable of reason. For man may be described as an animal, alive whilst subject to death, and endowed with reason. When therefore the highest is gone, why should the lower branches remain? How can the worth of any creature remain if virtue itself (the basis of well-being) be lost?
But in condemnation of the rest of men, and to manifest the goodness of God, it is written that Noah found grace in the eyes of the Lord. From these words we learn that the sin of others doth not cast its shadow on the righteous man, since Noah himself is preserved, to be the ancestor of the whole human race. He is praised, not because he was of a noble race, but because he was a just man and perfect. For the lineage of the upright man is to be traced in the pedigree of virtue which doth come forth from him. Even as blood maketh the lineage of man, so doth virtue form the lineage of souls. As the kindreds of men are made great by the grandeur of their lineage, so is the honour of souls made manifest by the grandeur of their virtues.
DOUAY-RHEIMS
The descendants of Seth, and Enos are here called the "The sons of God" from their religion and piety: whereas the ungodly race of Cain, who by their carnal affections lay groveling upon the earth, are called the children of men. The unhappy consequence of the former marrying with the latter, ought to be a warning to Christians to be very circuмspect in their marriages: and not to suffer themselves to be determined in their choice by their carnal passion, to the prejudice of virtue or religion.
Giants. It is likely the generality of men before the flood were of a gigantic stature in comparison with what men are now. But these here spoken of are called giants, as being not only tall in stature, but violent and savage in their dispositions, and mere monsters of cruelty and lust.