"the sons of God were angels"
Absolutely not. This is a fairytale and a doctrine of man. No serious teacher of the Word believed in the past that angels can have sex with women and produce children. I don’t know why this belief is gaining ground in our generation.
To understand better what happened in the beginning of Genesis 6, I recommend to first have a look at Genesis 5 to see some interesting things.
For example, that Adam and Eve had other sons and daughters after Seth was born ! (Genesis 5:4)
So mankind was made of 3 branches
1/ The branch of Seth
2/ The branch of Cain
3/ The branches of the brothers and sisters of Seth and Cain
Verse 1 in Genesis 6 say that men began to multiply.
The branch of Cain was the first to fall away.
Cain took a wife [ Genesis 4:17]
For obvious reasons, Cain’s wife was obviously not from Cain’s branch !
Cain’s wife was from one of the other branches , either from Seth’s branch , either from any of the other branches which are the other sons and daughters of Adam and Eve.
So we see that , rather quickly, one of the other branches became corrupted too, going the way of Cain !
Cain’s wife was probably one of his nieces or one of his sisters (if it was a niece, at this time men lived a long time and a difference of 50 years between spouses was not important , it would be like a difference of 6 years for us)
So Cain’s branch was the first corrupted, and another branch began to follow Cain's way quickly.
Seth‘s branch was the last to continue resist against corruption, as Noah the righteous was from Seth.
The second branch(es) to be corrupted were likely the other branches which were not from Cain’s lineage (which was already corrupted), nor from Seth’s.
But the fact that there was only one close family (from Seth’s branch) to be saved (8 persons) tells us that Seth’s branch was also corrupted !
In the time of the flood , where was all Noah’s nephews , nieces , grand nephews , and grand nieces ?
We know that Noah did not became a dad before being 500 (see Genesis 5) when all others before him, and likely the dads in the other branches too, engendered their first children at an average age of 100.
So while Noah had not children , the largest family of Noah (issued from his brothers and sisters) and all the rest of Seth’s lineage , and all the rest of mankind family were increasing in number.
From Noah’s generation , none other from the lineage of Seth remained in godliness.... !
When Seth’s lineage was supposed to keep the testimony of God !
So what we are seeing in this text of Genesis 6 is the falling away of ALL, Cain’s branch , the other branches , and even Seth ‘s branch.
What we know is that men began to multiply (Genesis 6:1). How did they multiply ? It could only be by the means of intermarriages between the 3 different branches of Adam !
Let the Holy Spirit of Truth shed light in what it is said in these texts ,
and not believe every wind of doctrine as these days are evil and false doctrines abound.
Some who believe that ‘the sons of God in that text are angels’ and that the Nephilim will return, like to quote Jesus when He says “as in the days of Noah”, when he did not said a thing about angels, but talked about marriage ! People were … marrying and being given in marriage [Luke 17:27]
Anyway they are right it is good to quote this verse, but not for the same reasons. For as in the days of Noah , we know that a great FALLING AWAY from the Truth and the faith is happening before His return.
The 144000 do not fall like the others. They have not compromised themselves with women, and in their mouths are no lie.
They have not compromised the Truth with churches that teach doctrines of men and demons. In other words , “They do not marry with the daughters of men”.
Noah is a type of these 144000.
After the last falling away , there is tribulation , so the multitudes of men and women who are perishing, have a better chance than in Noah’s generation.
What is written in Genesis 6 is not the breeding of 2 species, but the great falling away, where the sons of God learned the way of the godless.
So what caused the anomalies (giants) then ? Well there was 2 main curses in Genesis , one (the most important) in Eden after the fall of Adam , and another after the flood , with the curse of Ham (by Noah)
If you pay attention you will see that all giants in the times of Joshua , and later in the times of David, were descendants of Ham (and not from Seth and Japhet).
We know that curses can generate hereditary sickness( in those cases likely genetic anomalies involving the glands, anomalies remarkable enough to be referenced in the scriptures. It is highly probable they may have found favourable ground to develop themselves with the intermarriages).
The fate of these giants was unfortunate , for likely superstition was great in these times , the giants were 'men of renown' (possibly they were taken for 'demigods'), this would have increased even more violence , idolatry , and who knows what other sins.