First and foremost, the Bible is a spiritual book and secondly, God spoke in parables and without a parable he did not speak to them.
Most people read and critic the Bible like it's an ordinary, instead of a Spiritual and Holy Book!
God, in the beginning, created everything AFTER it's own kind, therefore, it is NOT possible for angels [spirit beings] to procreate with women kind [flesh and bones]!!!
And the "giants" or "sons of God" I will demonstrate with New Testament Scripture who they are.
The key verses in the Old Testament is Genesis 6:1-4 which reads:
1) And it came to pass when all men began to multiply on the face of the earth, and daughters were born unto them,
2) And the sons of God saw the daughters of men that they were fair [beautiful]; and they took them wives of all which they chose
, 3) 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.
4) There were giants in the earth in those days, and also after that, when the sons of God came in [sexual intimacy] unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown. [these same "mighty men", as we know them today are those of our "super athletes" and "celebrities" [men of renown].
As asserted, below you will find Scripture proof who the sons of God, denotes.
John 1:12-13; Romans 8:14; 19; Philippians 2:13; 1 John 3:1-2.
This whole matter of Genesis 6:4-7 lies in what 2 Corinthians 6:14 teaches: "Be ye not unequally yoked with unbelievers..."
But notice what God had to say in Genesis 6:5-7:
5) And God saw that the wicked of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
6) And it repented the LORD that he had made man on the earth, and it grieved him at his heart.
7) And the LORD said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.
To God Be The Glory