Genesis 6:4 says, "There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown."
To make thinks hazier, some bible versions mistakenly interpret the text instead of simply translating it. International Standard Version renders Genesis 6:4 as, "The Nephilim were on the earth at that time (and also immediately afterward), when those divine beings were having sexual relations with those human women, who gave birth to children for them. These children became the heroes and legendary figures of ancient times."
Who were the sons of God that married daughters of man? Some people speculate that they were angels, fallen angels, in particular, that mated with humans bringing about the birth of giants or Nephilim.
To answer this question, I like to emphasize 3 critical points:
1. Angels, fallen or unfallen, cannot produce an offspring with Humans
Can angels, fallen angels or divine beings, produce an offspring with mankind?
Genesis 1:24 says, "And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so."
Notice that the passage said that the earth should bring forth living creatures after his kind. A cattle cannot produce an offspring with an insect, neither would a bird with a fish. Should we suppose that a human being can produce an offspring with an angel? Man and angels are of different kinds. Psalms 8:5 says, "For thou hast made him a little lower than the angels, and hast crowned him with glory and honour."
Breeding a Lion with a Tiger will produce a Liger. It is possible to breed 2 different species of cats but we can never crossbreed a cat and a dog.
When God created Adam, Adam cannot bear children with other creatures. Because of this, God needed to create Eve. It was not until the creation of Eve that God told humans to be fruitful, multiply, and replenish the earth.
"And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth."
"So God created man in his own image, in the image of God created he him; male and female created he them."
"And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth."
Given these facts, we can clearly see that fallen angels cannot have children with humans. If the sons of God were able to mate with the daughters of man, the sons of God must be humans. This brings us to our next point.
2. The Sons of God in Genesis 6 were Men, not demons
It is true that the term Sons of God were used in the book of Job to describe beings that presented themselves with God: "Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them." (Job 1:6). When we consider Job 38:7 which says, "When the morning stars sang together, and all the sons of God shouted for joy...", we can say that at least in the book of Job "sons of God" refer to celestial beings. Nonetheless, we should notice that Satan was differentiated from those called Sons of God. The passage says, "the sons of God came to present themselves before the LORD, and Satan came also among them". Meaning, Satan or any demons cannot be considered sons of God. Sons of God are no demons at all.
In the first place, we already understood that angels, fallen or unfallen, cannot mate with man. So, who are the Sons of God?
It is important to notice that Adam was also called the Son of God. Luke 3:38 says, "Which was the son of Enos, which was the son of Seth, which was the son of Adam, which was the son of God." Given this fact, it would be appropriate to call Adam's descendants through his son Seth as "sons of God".
Genesis chapter 5 emphasized that God made Adam after His likeness and that Seth was born after the likeness and image of Adam:
"This is the book of the generations of Adam. In the day that God created man, in the likeness of God made he him... Male and female created he them; and blessed them, and called their name Adam, in the day when they were created... And Adam lived an hundred and thirty years, and begat a son in his own likeness, after his image; and called his name Seth..." -Genesis 5:1-3
On the contrary Cain, the murderous son of Adam, was described as a son of the wicked one. 1 John 3:12 says, "Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous."
Cain's descendants became lovers of worldly pleasures. Genesis 4:19-23 tells us that Cain's descendants became tent-dwellers and cattle herders, players of harp and organ, artificer of brass and iron, and murderous. These descriptions fit very well to what the Bible describes as sons of men:
"O ye sons of men, how long will ye turn my glory into shame? how long will ye love vanity, and seek after leasing? But know that the LORD hath set apart him that is godly for himself: the LORD will hear when I call unto him." - Psalms 4:2, 3
It is very interesting to note that a daughter named Naamah was mentioned in the genealogy of Cain. Genesis 4:22 says, "And Zillah, she also bare Tubalcain, an instructer of every artificer in brass and iron: and the sister of Tubalcain was Naamah." The name Tubalcain means "offspring of Cain" and the name Naamah means " beautiful, agreeable, lovely." Very interesting? Yes! Because this reminds me of Genesis 6:2, "That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose." The word fair here was translated from the Hebrew word "tobe" which means beautiful, good, and loving.
Genesis chapter 4 describes the genealogy of Cain and while chapter 5 describes the genealogy of Seth. Then, chapter 6 goes on and said:
"And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose." -Genesis 6:1, 2
Therefore, the "sons of God" mentioned in Genesis 6:2 are nothing more than the descendants of Seth.
If the parents of the giants were the descendants of Seth and the descendants of Cain. The giants must be nothing more than giant men. This brings us to our next point:
3. The giants or Nephilim were called "man" in Genesis 6
In Genesis 6:3, 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." Notice that God said that he cannot strive with man, man not demons. Then verse 4 proceeds, "There were giants (Nephilim in Hebrew) in the earth in those days..." In its proper context, we can clearly see that God described the giants as men, not demons. The verse continues, "and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown." Once again, the giants were called men, mighty men, and men of renown.
You might be thinking, how can ordinary men give birth to giants? Simple genetics is the answer. Remember the Lion and the Tiger? Yes. Lions and Tigers are of different species but are of the same genus. When the two separate species were bred a much bigger offspring was produced. This happens because of what is called "hybrid vigor". When superior genes from 2 ordinary species come together they would result in a superior species. The same thing happened to Wolfdogs. Yes! That's right! A Wolfdog is a larger species produced when mating a domestic dog and a wolf.