In Genesis 4, we see the line of Cain:
Gen 4:17 Cain knew his wife, and she conceived and bore Enoch. When he built a city, he called the name of the city after the name of his son, Enoch.
Gen 4:18 To Enoch was born Irad, and Irad fathered Mehujael, and Mehujael fathered Methushael, and Methushael fathered Lamech.
Gen 4:19 And Lamech took two wives. The name of the one was Adah, and the name of the other Zillah.
Gen 4:20 Adah bore Jabal; he was the father of those who dwell in tents and have livestock.
Gen 4:21 His brother's name was Jubal; he was the father of all those who play the lyre and pipe.
Gen 4:22 Zillah also bore Tubal-cain; he was the forger of all instruments of bronze and iron. The sister of Tubal-cain was Naamah.
Gen 4:23 Lamech said to his wives: “Adah and Zillah, hear my voice; you wives of Lamech, listen to what I say: I have killed a man for wounding me, a young man for striking me.
Gen 4:24 If Cain's revenge is sevenfold, then Lamech's is seventy-sevenfold.” (ESV)
Lamech was evil, in the way of Cain but much worse. And, yes, that contrasts with Seth's line:
Gen 4:25 And Adam knew his wife again, and she bore a son and called his name Seth, for she said, “God has appointed for me another offspring instead of Abel, for Cain killed him.”
Gen 4:26 To Seth also a son was born, and he called his name Enosh. At that time people began to call upon the name of the LORD.
Gen 5:22 Enoch walked with God after he fathered Methuselah 300 years and had other sons and daughters.
Gen 5:23 Thus all the days of Enoch were 365 years.
Gen 5:24 Enoch walked with God, and he was not, for God took him.
Gen 5:25 When Methuselah had lived 187 years, he fathered Lamech.
Gen 5:26 Methuselah lived after he fathered Lamech 782 years and had other sons and daughters.
Gen 5:27 Thus all the days of Methuselah were 969 years, and he died.
Gen 5:28 When Lamech had lived 182 years, he fathered a son
Gen 5:29 and called his name Noah, saying, “Out of the ground that the LORD has cursed, this one shall bring us relief from our work and from the painful toil of our hands.” (ESV)
Gen 6:8 But Noah found favor in the eyes of the LORD.
Gen 6:9 These are the generations of Noah. Noah was a righteous man, blameless in his generation. Noah walked with God. (ESV)
Right from the start Cain's line was evil and Seth's line "began to call upon the name of the LORD." Note the repetition in 5:22 and 5:24, and then the very same is said of Noah in 6:9.
Notice where chapter 6 starts, after giving lineages of Cain and Seth in the preceding two chapters, which I have provided:
Gen 6:1 When man began to multiply on the face of the land and daughters were born to them,
Gen 6:2 the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. (ESV)
What we should note here is that this is not really "when man began to multiply," as they had clearly been multiplying for well over a thousand years, but rather it could be speaking of a specific group of people, namely, Cain's lineage or of an increase in multiplying generally. When we get to verse 2--"the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose."--we have as the immediate context the lineages of Cain and Seth as well as at least one of them multiplying.
While "sons of God" is used of angels in the OT, that is fraught with difficulty, not the least of which is the apparent inability of angels to procreate, according to Jesus (Matt. 22:30). That Jesus says "angels in heaven" is irrelevant as to whether those angels are elsewhere. To suggest that angels "on earth" can multiply and those in heaven cannot, is rather nonsensical. It would also suggest that they not only have the equipment to do so but also the genetic ability to do so, which means that angels have the creative power that God alone has--the ability to create ex nihilo--as they would have to be able to create for themselves all that is necessary. But, that is to go far beyond Scripture.
Rather, we should look elsewhere in the Bible to see if there is another legitimate understanding of "sons of God." And so we find the following:
Deu 32:4 “The Rock, his work is perfect, for all his ways are justice. A God of faithfulness and without iniquity, just and upright is he.
Deu 32:5 They have dealt corruptly with him; they are no longer his children because they are blemished; they are a crooked and twisted generation.
Deu 32:6 Do you thus repay the LORD, you foolish and senseless people? Is not he your father, who created you, who made you and established you? (ESV)
Quite significant that those who are faithful to God are his children and those who are corrupt, "blemished," and "crooked and twisted" are "no longer his children."
Psa 73:15 If I had said, “I will speak thus,” I would have betrayed the generation of your children. (ESV)
Psa 82:6 I said, “You are gods, sons of the Most High, all of you;
Psa 82:7 nevertheless, like men you shall die, and fall like any prince.” (ESV)
The above is speaking of judges who were God's representatives but chose to judge unjustly, and so they would die "like men."
Hos 1:10 Yet the number of the children of Israel shall be like the sand of the sea, which cannot be measured or numbered. And in the place where it was said to them, “You are not my people,” it shall be said to them, “Children of the living God.” (ESV)
So, we can clearly see that "sons of God" can legitimately refer to those who are godly, those who are God's representatives on earth. In the case of Gen. 6:2, "the sons of God" are, or can legitimately be, those who "call upon the name of the LORD."
Gen 6:4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. (ESV)
Similar to verse 1, "The Nephilim were on the earth in those days . . . when the sons of God came in to the daughters of man and they bore children to them," does not mean that the Nephilim were the offspring, but can legitimately be understood as saying that "the Nephilim were [already] on the earth in those days." That is, the Nephilim are not the offspring of "the sons of God" and "the daughters of man."