That is because you did not follow the context of the geneologies. We are going to base our interpretation of the Word of God on what we think to fit our theology. Can you show me were there is the introduction of beings other than humans in the context of Genesis 1:26-5:32?
When Genesis 6:1, 3, 5-50:26 stays with the context of humans, that would make 6:2, 4 be life forms other than human in the context of all of Genesis.
Genesis 4:16-24 “Then Cain went out from the presence of the Lord and dwelt in the land of Nod on the east of Eden.17 And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son—Enoch.18 To Enoch was born Irad; and Irad begot Mehujael, and Mehujael begot Methushael, and Methushael begot Lamech.19 Then Lamech took for himself two wives: the name of one was Adah, and the name of the second was Zillah.20 And Adah bore Jabal. He was the father of those who dwell in tents and have livestock.21 His brother’s name was Jubal. He was the father of all those who play the harp and flute.22 And as for Zillah, she also bore Tubal-Cain, an instructor of every craftsman in bronze and iron. And the sister of Tubal-Cain was Naamah.
23 Then Lamech said to his wives: “Adah and Zillah, hear my voice; wives of Lamech, listen to my speech! For I have killed a man for wounding me, even a young man for hurting me.24 If Cain shall be avenged sevenfold, then Lamech seventy-sevenfold.”
The context of Genesis 4:16-24 is Cain's family that had 8 sons that were patriarchs in Cain's geneology, with the mention of one females being the sister of Tubal-Cain how was Naamah, she was not even mentioned as being a daughter born to Lamech, no one other than human beings in that geneology.
Genesis 4:25-5:32 is the geneology of Adam, with a son being born to him as the new patriarch to Adam and Seth had a son as well. Genesis 4:25-26 “And Adam knew his wife again, and she bore a son and named him Seth, “For God has appointed another seed for me instead of Abel, whom Cain killed.”26 And as for Seth, to him also a son was born; and he named him Enosh. Then men began to call on the name of the Lord.”
Notice that after Enosh's brith, "men began to call on the name of the Lord" what is interesting is that in 5:24 we see that one of Adam's patriarchs walked with God andwas taken, he could of walked with God as Adam and Eve did in the garden, here the verse. “And Enoch walked with God; and he was not, for God took him.” Now we can look back to Cain's geneology and one of his patriarchs, Lamech followed in the foot-steps of Cain. Genesis 4:23-24 “Then Lamech said to his wives: “Adah and Zillah, hear my voice; wives of Lamech, listen to my speech! For I have killed a man for wounding me, even a young man for hurting me.24 If Cain shall be avenged sevenfold, then Lamech seventy-sevenfold.”
When you read Genesis 4:16 and 4:26 we see that the two geneologies are going in two different directions, Genesis 4:16 “Then Cain went out from the presence of the Lord and dwelt in the land of Nod on the east of Eden.” Genesis 4:26 “And as for Seth, to him also a son was born; and he named him Enosh. Then men began to call on the name of the Lord.” With these verses we see that the two geneologies were headed in different directions, with Adam's geneology birthing many children and the mention of many daugthers.
Genesis 5:4 “After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.” 5:7 “After he begot Enosh, Seth lived eight hundred and seven years, and had sons and daughters.” 5:10 “After he begot Cainan, Enosh lived eight hundred and fifteen years, and had sons and daughters.” 5:13 “After he begot Mahalalel, Cainan lived eight hundred and forty years, and had sons and daughters.” 5:16 “After he begot Jared, Mahalalel lived eight hundred and thirty years, and had sons and daughters.” 5:19 “After he begot Enoch, Jared lived eight hundred years, and had sons and daughters.” 5:22 “After he begot Methuselah, Enoch walked with God three hundred years, and had sons and daughters.” 5:26 “After he begot Lamech, Methuselah lived seven hundred and eighty-two years, and had sons and daughters.” 5:30 “After he begot Noah, Lamech lived five hundred and ninety-five years, and had sons and daughters.”
Go back up to Cain's geneology and there is no mention of any daughter being born to his geneology, but reading through the context of Genesis 5 we read of 9 different patriarchs having daughters born to them. Genesis 6:1 “Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them,”
Now what came to pass? "When men began to multiply on the face of the earth" Who's geneology had 8 sons born to him with no mention of sons and daughters being born to them? Cain's. Now who's geneology has no mention of a daugter being born to his patriachs? Cain's, so in that context of Genesis 4:16-5:32 who would be the ones that are multiplaying on the face of the earth “and daughters were born to them,”? There is only one genealogy that has no mention of daughters being born to them, with only 8 sons being born to him and that is Cain. Now that Cain's genealogy is multiplying on the face of the earth and daughters being born to him now, who does that leave on earth that could take Cain's daughters as wives? Adam's genealogy who are called the sons of God only because they had been the ones that started to call on the name of the Lord. Those titles were only to identify the two different genealogies because of how they had been living before wickedness fill their hearts. It is the only logical conclusion that one group was Cain's genealogy and the other was Adam's genealogy, there are no other groups of mankind on the face of the earth.
The nephilim are mentioned in 6:4, the root word for nephilim of "to fall" and when we read 6:5 “Then the Lord saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually.” We can conclude that many men have fallen, from both genealogies, yes Lamech killed a man and Cain and his offspring dwelt outside the presents of the Lord, but Genesis 4:26 does not limit the men that called on the name of the Lord as Adam's genealogy only. But by this point “the Lord saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually.”
Also notice here the change of wording from "men" that were multiplying on the face of the earth, "the wickedness of man was great in earth, and that every intent of the thought of his heart was only evil continually." Men was a group that was multiplying and man is all of mankind or humans, it fits the context of the Scripture.