You all realize that it is just a story, right? I mean, it is not real, OK?
With that in mind, let’s look to the story to see who Cain may have married.
The vast majority of answers center around the other sons and daughters of Adam and Eve, requiring the human race to have been produced through incest.
This can be avoided by a more liberal interpretation of the Genesis story. You
will recall there are two versions of God creating Man.
In the first, Gen.- 1: 26, “And God said, Let us make man in our own image ; after our likeness, and let them have dominion over…”
Gen, 1: 27, “So God created man in his own image, in the image of God created he him; male and female created he them.”
Gen. 1: 28, “ And God blessed them, and God said unto them, Be fruitful and multiply…”
There is no indication of individuality as there is in the second version, in Gen. 2.
This could very well be the accounting of God creating Mankind, sending a mass of humans out into the world to ‘replenish the earth’.
Besides, in this version God told ‘mankind’ that it was Ok to eat the fruit of
any tree, no garden of Eden, no tree of the knowledge of good and evil. God was speaking to the general populace, which he created and sent out to ‘be fruitful and multiply.
One of the reasons for the story was to show where the Hebrews came from. (It is the Hebrew God, after all) Being the “chosen people”, they did not come from the general populace, but rather, from God’s special pair, Adam and Eve.
In the second version, God creates Adam, an individual, to tend God’s garden. (God don’t pull no weeds.)
And almost as an afterthought, creates Eve to be ‘an help meet’.
The people that Cain encounters, from which he selected his bride, are those already released into the world by God, in Gen. 1. This avoids the implication of incest so carefully sidestepped by apologists with their, oh it was OK back then, and yeah, DNA was more pure, and blah, blah, blah.
Bottom line: It really was only a story…