Let me modify my own speculation. I wrote:
Remember that almost proverbial question: "who did Cain marry?" Unless YHWH created more women than just Eve, he married his own sister. There was no one else.
Possibly (again, speculation) if Abel was married and had kids before he was murdered, Cain could also have married his own niece(s) as well as his own sister(s). Likewise for Abel and Seth possibly marrying nieces.
Someone had to marry his own sister. And it had to occur long before Seth was born, or else there were only Adam, Eve, Cain, Seth, and Abel's grave until Adam and Eve had those sons and daughters mentioned in Genesis 5:4, in which case the world must have been a lonely place...
Genesis 5:4 NKJV After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.
Did Adam and Eve wait 130 years to obey this command?
Genesis 1:28 NKJV Then God blessed them, and God said to them, "Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth."
It's hard to imagine the most perfect man and most perfect woman in all history (talking about most perfect physically, not spiritually - spiritually, that would be Yeshua) having only three children in 130 years of marriage. And if Adam took other wives from among his female descendants, there could have been quite a population explosion taking place prior to Genesis 5:4, because his male descendants would have followed his example. Scripture rarely records the birth, death, etc., of specific individuals whose lives are not directly relevant to the topic being written about.
Did the genealogy of Cain in chapter 4 precede or follow (chronologically) that of Adam in chapter 5? IMHO, the events in Genesis are written in chronological order, possibly with some overlap. Lamech could have been born to Methushael's wife either
before or after Seth was born to Adam's wife (Seth's mother might not have been Eve...), but IMHO, it was before. But the
only clue we have to any such chronology is the order in which the events are recorded in the Genesis account.
There are four generations from Cain to Lamech. But Lamech's ancestor Enoch most probably was born soon after the murder of Abel, long before Seth was born. That would mean that Cain had a woman from somewhere before Genesis 5:4.
But remember - we are not told how old Adam was when Cain and Abel were born, nor are we told how old Cain and Abel were when the first murder took place.
And one final (for this post) thought on the matter: YHWH did NOT directly create any women other than Eve, because Eve is called the mother of all living:
Genesis 3:20 NKJV And Adam called his wife's name Eve, because she was the mother of all living.
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All this speculation might be fun for some of us, but what difference does it make? How many angels can dance on the head of a pin...?