Shalom, sojourner4Christ.
From the above Scriptures in Genesis 4, we learn that Qayin and his family went underground ... LITERALLY. Qayin was cursed "FROM the earth"; he would be a "fugitive and a vagabond IN the earth," and he was driven out "FROM the (sur)face of the earth." Thus, the land of "Nowd" (which means "wandering") was a CAVERN SYSTEM! And, it was there that Qayin built the first city, Chanokh (Enoch), named after his son. His family stayed with him and he was their patriarch.
It wasn't until Lemekh killed in self defense that they emerged from that city. Despite his claim that he should be avenged "seventy and sevenfold," GOD SAID NOTHING! He had NO MARK like Qayin did! Revengers of blood, then, might not fear to take his life, and I believe that his fear drove him and his family from Chanokh in Nowd. His son, Yaval (Jabal), for instance was the first to live in tents. One might ask, "Why was he the first?" SIX generations after Qayin? Having lived for all that time away from the surface of the earth and out of the light of the sun (hidden from "God's face"), their skin may have been sensitive to the light and heat of the sun. Another son, Tuval-Qayin (Tubal-cain), Yaval's half-brother, was the first smith. Where did he develop such a love for metals?
His sister, Na`amah, is traditionally held by Jews to be the wife of Noach (Noah), which they believe is why she is mentioned at all.
Now, consider the bane of all humanity ... PREJUDICE. After six generations of Qayin's line being safely tucked away in Chanokh within Nowd, these men and their daughters emerge from the caves. The line of Shet (Seth) were "allowed," perhaps "ENTITLED," to live in the sun - under the face of God! THEY would consider themselves privileged, "SONS of God," and the line of Qayin would be perceived to be INFERIOR, mere sons and "daughters of men!" Reckoning themselves to be "entitled sons of God," they could freely TAKE these "daughters of men" as their wives, as any "noble" would be so entitled to do, and human trafficking began! They ENSLAVED these women, treating them as inferior chattel! This, and the fact that these bullies were PRAISED and RENOWNED for doing this, was "the straw that broke the camel's back" and caused God to say that He's had enough! Then, 120 years later, God sent the Flood.
Genesis 6:8
8 V-Noach maatsaa’ cheen b`eeyneey YHWH.
JPS Interlinear Bible
"And-Noah attained kindness in-eyes of-YHWH."
This should not be read as Noach attaining kindness from YHWH, but that YHWH found kindness attained by Noach! He treated Na`amah well! He treated his wife right! And, God preserved Noach and his family.
Now, granted. This is conjecture; HOWEVER, it does no damage to the text and it answers several questions at once! It also fits ALL the facts that have been given to us in the first six chapters of the Bible.
AND, it's a MUCH better solution than "angels interbreeding with men," which I feel is DEFINITELY a stretch of the imagination!