There is a misconception (pun intended) that angels, fallen or otherwise, are able to have sexual relations with humans and that this actually has happened in the past. Many fine Christians have been bamboozled by this simple mistake. The mistake is a result of misunderstanding the context of Genesis Chapter 6 and the results of believers marrying unbelievers and having children.
Identifying the sons of God
However in the book of Job Chapter One the NIV turns the sons of God into angels.
Job 1: 6 One day the angels[a] came to present themselves before the LORD, and Satan[b] also came with them. 7 The LORD said to Satan, “Where have you come from?”
So instead of fixing the wrong interpretation in Job they try to reconcile their error in Genesis! The NIV is not the only version that does this. Here is what the New Century Version says:
The Contemporary English Version:
Genesis 6:1-2More and more people were born, until finally they spread all over the earth. Some of their daughters were so beautiful that supernatural beings [a] came down and married the ones they wanted. 3Then the LORD said, "I won't let my life-giving breath remain in anyone forever. [b] No one will live for more than one hundred twenty years." [c] 4The children of the supernatural beings who had married these women became famous heroes and warriors. They were called Nephilim and lived on the earth at that time and even later.
The Common English Version:
Genesis 6: 1 When the number of people started to increase throughout the fertile land, daughters were born to them. 2 The divine beings saw how beautiful these human women were, so they married the ones they chose. 3 The LORD said, “My breath[a] will not remain in humans forever, because they are flesh. They will live one hundred twenty years.” 4 In those days, giants[b] lived on the earth and also afterward, when divine beings and human daughters had sexual relations and gave birth to children. These were the ancient heroes, famous men.
The Good News Translation:
Genesis 6: 1 When people had spread all over the world, and daughters were being born,2 some of the heavenly beings[a] saw that these young women were beautiful, so they took the ones they liked.3 Then the Lord said, I will not allow people to live forever; they are mortal. From now on they will live no longer than 120 years.4 In those days, and even later, there were giants on the earth who were descendants of human women and the heavenly beings. They were the great heroes and famous men of long ago.
The Holman Christian Standard Bible implies the sons of God were other than human in the footnotes.
Genesis 6:1 When mankind began to multiply on the earth and daughters were born to them, 2 the sons of God saw that the daughters of man [a] were beautiful, and they took any they chose as wives [b] for themselves. 3 And the LORD said, "My Spirit will not remain [c] with [d] mankind forever, because they are corrupt. [e] Their days will be 120 years." 4 The Nephilim [f] were on the earth both in those days and afterwards, when the sons of God came to the daughters of man, who bore children to them. They were the powerful men of old, the famous men. [a] Genesis 6:2 Or the human women [b] Genesis 6:2 Or women [f] Genesis 6:4 Possibly means "fallen ones"; traditionally giants; [Nm 13:31-33]
Here is why it is important to understand the identity of the sons of God. It has to do with understanding our salvation and our relationship with God. I will be quoting scripture from the New American Standard because it doesn't use “inclusive language”.
Hebrews 1 1 God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, 2 [a]in these last days has spoken to us [b]in His Son, whom He appointed heir of all things, through whom also He made the [c]world. 3 [d]And He is the radiance of His glory and the exact representation of His nature, and [e]upholds all things by the word of His power. When He had made purification of sins, He sat down at the right hand of the Majesty on high, 4 having become as much better than the angels, as He has inherited a more excellent name than they.
5 For to which of the angels did He ever say,
“YOU ARE MY SON,
TODAY I HAVE BEGOTTEN YOU”?
“I WILL BE A FATHER TO HIM
AND HE SHALL BE A SON TO ME”?
6 And [f]when He again brings the firstborn into [g]the world, He says,
“AND LET ALL THE ANGELS OF GOD WORSHIP HIM.”
7 And of the angels He says,
“WHO MAKES HIS ANGELS WINDS,
AND HIS MINISTERS A FLAME OF FIRE.”
8 But of the Son He says,
“YOUR THRONE, O GOD, IS FOREVER AND EVER,
AND THE RIGHTEOUS SCEPTER IS THE SCEPTER OF [h]HIS KINGDOM.
9 “YOU HAVE LOVED RIGHTEOUSNESS AND HATED LAWLESSNESS;
THEREFORE GOD, YOUR GOD, HAS ANOINTED YOU
WITH THE OIL OF GLADNESS ABOVE YOUR COMPANIONS.”
“YOU, LORD, IN THE BEGINNING LAID THE FOUNDATION OF THE EARTH,
AND THE HEAVENS ARE THE WORKS OF YOUR HANDS;
11 THEY WILL PERISH, BUT YOU REMAIN;
AND THEY ALL WILL BECOME OLD LIKE A GARMENT,
12 AND LIKE A MANTLE YOU WILL ROLL THEM UP;
LIKE A GARMENT THEY WILL ALSO BE CHANGED.
BUT YOU ARE THE SAME,
AND YOUR YEARS WILL NOT COME TO AN END.”
13 But to which of the angels has He ever said,
“SIT AT MY RIGHT HAND,
UNTIL I MAKE YOUR ENEMIES
A FOOTSTOOL FOR YOUR FEET”?
14 Are they not all ministering spirits, sent out to render service for the sake of those who will inherit salvation?
How then can the sons of God be angels? Hebrews 2 says more about the relationship of God, man and angels:
Hebrews 2: 5 For He did not subject to angels [g]the world to come, concerning which we are speaking. 6 But one has testified somewhere, saying,
“WHAT IS MAN, THAT YOU REMEMBER HIM?
OR THE SON OF MAN, THAT YOU ARE CONCERNED ABOUT HIM?
7 “YOU HAVE MADE HIM [h]FOR A LITTLE WHILE LOWER THAN THE ANGELS;
YOU HAVE CROWNED HIM WITH GLORY AND HONOR,
[i]AND HAVE APPOINTED HIM OVER THE WORKS OF YOUR HANDS;
8 YOU HAVE PUT ALL THINGS IN SUBJECTION UNDER HIS FEET.”
For in subjecting all things to him, He left nothing that is not subject to him. But now we do not yet see all things subjected to him.
Hebrews explains that it is through Jesus that God will give man the inheritance.
Galatians 3: 26 For you are all sons of God through faith in Christ Jesus.
Romans 8:12 So then, brethren, we are under obligation, not to the flesh, to live according to the flesh— 13 for if you are living according to the flesh, you [f]must die; but if by the Spirit you are putting to death the deeds of the body, you will live. 14 For all who are being led by the Spirit of God, these are sons of God. 15 For you have not received a spirit of slavery [g]leading to fear again, but you have received [h]a spirit of adoption as sons by which we cry out, “Abba! Father!” 16 The Spirit Himself testifies with our spirit that we are children of God, 17 and if children, heirs also, heirs of God and fellow heirs with Christ, if indeed we suffer with Him so that we may also be glorified with Him.
18 For I consider that the sufferings of this present time are not worthy to be compared with the glory that is to be revealed to us. 19 For the anxious longing of the creation waits eagerly for the revealing of the sons of God. 20 For the creation was subjected to futility, not willingly, but because of Him who subjected it, [i]in hope 21 that the creation itself also will be set free from its slavery to corruption into the freedom of the glory of the children of God. 22 For we know that the whole creation groans and suffers the pains of childbirth together until now. 23 And not only this, but also we ourselves, having the first fruits of the Spirit, even we ourselves groan within ourselves, waiting eagerly for our adoption as sons, the redemption of our body. 24 For in hope we have been saved, but hope that is seen is not hope; for who hopes for what he already sees? 25 But if we hope for what we do not see, with perseverance we wait eagerly for it.
You see the sons of God are people who have a relationship with God. In the Old Testament that relationship was established through faith in a coming salvation; in the New Testament the relationship is established in Christ, who is the salvation and will finally accomplish our physical salvation when He returns for the resurrection.
We have seen much evidence that the average human before the Flood was taller than the average human today. Human trackways under Australian rock layers had a stride that on average would have indicated a height of 6'4" but much larger humans certainly existed. Large teeth, femurs and fingers as well as very large individual tracks have been found that indicate that people from ten to sixteen feet tall lived before the Flood.
But that should be no surprise to anyone. Dinosaurs were huge before the Flood and the massive animals could not have survived in today's atmosphere. The amount of oxygen in the atmosphere would not support a dinosaur longer than 100 feet or a Pterodactyl with a wingspan of 52 feet!
Crocodiles, dragonflies, creatures of all kinds which are found both in the fossil records and also alive today are smaller than their ancestors who were preserved in fossil rocks. Giant people (from our point of view) living in those days absolutely existed, we have the fossil evidence to demonstrate it. The descriptions of Goliath and his brothers, the sons of Anak and the description of King Og, all from the Bible, indicate that some very large throwback humans were around after the Flood. Even today there are people who exceed seven feet tall, some of them not tall because of a glandular disorder but simply by heritage.
Very large humans lived and very large animals lived. However, that they lived is in no way indicative that humans ever mated with spirit beings. Both Cain and Seth were born of Adam and Eve. Cain was not the son of Satan. The Nephilim were not children of angels, fallen or otherwise. That is completely illogical and Biblically unsupportable!