The Bible quotes that the “sons of God” married the “daughters of men” and that man “he also is flesh”:
Genesis 6:1-3 King James Version (KJV)
1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
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.
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.
The “sons of God” are simply God’s people, those who are blameless and harmless, so fallen Angels or Devils can’t be called sons of God:
Philippians 2:15 King James Version (KJV)
15 That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world;
Furthermore, Jesus mentioned that Angels do not marry:
Matthew 22:30 King James Version (KJV)
30 For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
In fact, God made Angels as “spirits” and a “flame of fire”:
Hebrews 1:7 King James Version (KJV)
7 And of the angels he saith, Who maketh his angels spirits, and his ministers a flame of fire.
Eli’s sons who did not follow the Lord but did whatever they wanted, were known as “sons of Belial” instead of sons of God:
1 Samuel 2:12 King James Version (KJV)
12 Now the sons of Eli were sons of Belial; they knew not the Lord.
The daughters of men are the daughters of the “sons of men” with hearts that are “full of evil and madness”:
Ecclesiastes 9:3 King James Version (KJV)
3 This is an evil among all things that are done under the sun, that there is one event unto all: yea, also the heart of the sons of men is full of evil, and madness is in their heart while they live, and after that they go to the dead.
Quite simply, Cain and his descendants departed “from the presence of the Lord,” and so these are the sons of men, and not sons of God:
Genesis 4:16 King James Version (KJV)
16 And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden.
From Seth’s descendants, men began “to call upon the name of the Lord”, and so would be called the sons of God:
Genesis 4:26 King James Version (KJV)
26 And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the Lord.
Before the flood there were giants, and even after the flood:
Genesis 6:4 King James Version (KJV)
4 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.
After the flood Ham had a son called Canaan:
Genesis 9:18 King James Version (KJV)
18 And the sons of Noah, that went forth of the ark, were Shem, and Ham, and Japheth: and Ham is the father of Canaan.
Moses instructed the people to “spy out the land of Canaan”:
Numbers 13:17 King James Version (KJV)
17 And Moses sent them to spy out the land of Canaan, and said unto them, Get you up this way southward, and go up into the mountain:
The spies told Moses that all the people of the land of Canaan are “men of a great stature”:
Numbers 13:32 King James Version (KJV)
32 And they brought up an evil report of the land which they had searched unto the children of Israel, saying, The land, through which we have gone to search it, is a land that eateth up the inhabitants thereof; and all the people that we saw in it are men of a great stature.
Among the Canaanites who were men of a great stature, were the “giants, the sons of Anak”:
Numbers 13:33 King James Version (KJV)
33 And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.
Arba was the father of Anak:
Joshua 15:13 King James Version (KJV)
13 And unto Caleb the son of Jephunneh he gave a part among the children of Judah, according to the commandment of the Lord to Joshua, even the city of Arba the father of Anak, which city is Hebron.
Arba was a great man among the Anakim and even had a city named after him called “Kirjatharba” but later called Hebron:
Joshua 14:15 King James Version (KJV)
15 And the name of Hebron before was Kirjatharba; which Arba was a great man among the Anakims. And the land had rest from war.
Kirjatharba is in the land of Canaan:
Genesis 23:2 King James Version (KJV)
2 And Sarah died in Kirjatharba; the same is Hebron in the land of Canaan: and Abraham came to mourn for Sarah, and to weep for her.
Abraham directed his servant not to get a wife for Isaac of the daughters of men or the “daughters of the Canaanites”:
Genesis 24:2-4 King James Version (KJV)
2 And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh:
3 And I will make thee swear by the Lord, the God of heaven, and the God of the earth, that thou shalt not take a wife unto my son of the daughters of the Canaanites, among whom I dwell:
4 But thou shalt go unto my country, and to my kindred, and take a wife unto my son Isaac.
Even Isaac mandated his own son Jacob not to intermarry with the daughters of Canaan:
Genesis 28:1-2 King James Version (KJV)
28 And Isaac called Jacob, and blessed him, and charged him, and said unto him, Thou shalt not take a wife of the daughters of Canaan.
God forbid intermarriage between His people, and the other evil nations:
Deuteronomy 7:3 King James Version (KJV)
3 Neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son.
The “sons of God” are simply His people, those who are “blameless and harmless”, “without rebuke” so fallen angels or devils can’t be called sons of God:
Philippians 2:15 King James Version (KJV)
15 That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world;
And the daughters of men are the daughters of the “sons of men” who have hearts that are “full of evil”:
Ecclesiastes 9:3 King James Version (KJV)
3 This is an evil among all things that are done under the sun, that there is one event unto all: yea, also the heart of the sons of men is full of evil, and madness is in their heart while they live, and after that they go to the dead.