That’s the basis of some branches of Christian Identity doctrine – see link in post #14. (It would be interesting if they got it from the Talmud . . .)
They mated with each other.
With men, this is impossible, but with God, all things are possible.
-FrL-
Other versions name Cain’s sister/wife as Luluwa, which is interesting in that it’s related to Lilith (the L-l-) and Eve (Hebrew - Chava.
Yes, Adam & Eve would know what children were. They were morally innocent at the time, not stupid or ignorant.
The command is here -
Genesis 2:
23 And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man.
24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
The clear implication is, when finding a mate, one’s parents (and conversely, one’s children) are off-limits.
IF there were indeed non-Adamic, non-spiritual peoples they may or may not have survived the Flood, which was mainly to wipe out those corrupt people who had “the Breath of Life”, the Divine spark breathed into Adam.
Can you imagine Genesis with Frazier and Miles instead of Cain and Abel?
If we’re going to just make stuff up can we make up that homosexuals are cool, and anyone who preaches on TV is the Devil?
*Will & Grace *already did that.
And it was Adam and Steve. 
And so did Adam cleave onto Steve. And he was the first power top.
The girls from Nod were my people. Over time the d became the tt of Nott. 
I am a Notty boy, as was my father, and his father before him.
Ahh, the German pronunciation.
Though in some versions Steve went by the name Lulu-Wowboy.
Personally, I’ve found a great many marriageable women in the land of Nod. But they always seem to be gone when I wake up.
How would they know? Did they play Snake and Apple before the snake and the apple? If you’re the only two people around, how would you know what a kid was if you never saw any? And if they were morally innocent, does that mean they didn’t make love till they left the Garden?
Eh, that just took me right back - and sometimes I was really unhappy to have woken up, too. 
I wish they all could be land of Nod girls,
I wish they all could be land of Nod girls,
I wish I could know the sweet girls from Nod.
I have trouble believing that Adam and Eve were the only people God created. First, sure, but not only. Genesis 4 tells us that Eve had Cain, and then Able. Genesis is very methodically about the whole begetting thing, so why would other children be left out?
And once Cain kills able, it says "“Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me. 15: And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.”
Who are these people who’ll find and kill him? He doesn’t seem to have any more siblings until “And 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 bare Enoch — 25 : And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew.”
If we are to assume that those people in Nod are Adam and Eve’s unmentioned kids (and why would they be unnamed? Enoch and Seth don’t seem to do much compared to their older brothers, and they’re named) that’d mean that Cain and Able were tremendous slackers compared to their unnamed younger siblings that were off building a town.
And why was Seth a notable replacement for Able if they’d had a lot of kids we don’t know by name? (actually, I can’t figure out why Enoch isn’t “the replacement” since he was born next.)
I don’t know, I have more difficulty believing that the younger siblings, at least some of them sisters, were off building a city while Cain and Able farmed for Mom and Dad than I do believing that God created some people in Nod too, but we don’t learn about them because they didn’t do anything considered too imporatant by the scribes who wrote out the bible.
I’ve probably mentioned this here before, but I tend to believe along lines similar to this ever since my stepfather (a devout Christian who happens to believe that much of the Old Testament is allegorical, not literal) explained his theory to me. Basically, “Adam” was not a “singular male”, but rather “Adam” was “mankind”. The evidence for this comes from the verse that says “… male and female created he them” (sorry, don’t have the exact verse reference handy). Basically, it says God created Adam, and calls Adam “them”.
That idea goes a long way toward explaining those exceptional lifespans, as well, if you consider that each name given represents a particular civilization or tribe. In other words, the original tribe of mankind, “Adam”, lasted for 930 years and produced “Cain”, “Abel”, and “Seth” (among others) - these “sons” were actually groups that eventually split off from the main group — possibly due to things like regional overpopulation — and set up camp someplace else. “Cain” murdering “Abel” could easily be an allegorical way of describing one of these offshoot tribes going to war against another.
As for “Eve”, my stepfather believes that “she” represents God gifting Adam/mankind with sentience/freewill, rather than “Eve” being a “singular female”.
Wait, I do know this one after all. Enoch is Cain’s son, not his brother, so Seth is Adam and Eve’s third kid. Woops.