Are you really setting up "popularity" as the arbiter of truth as a doctrine?
Are you Catholic? I would have to say that their doctrines are the most popular on Earth even today.
No, your doctrine (and that is the problem, it actually *is* doctrine for you) is based solely on the NEED for the doctrine to be correct in order to fit your other readings.
But here is what it comes down to:
Yes, absolutely, the account of Adam being made as a special creation is, indeed, the plainest simplest reading. There is no doubt about that. And it might very well be true. If read alone, without bringing in any other "context", whether it be other Scripture, our knowledge from our study of the natural world, etc, there is no doubt that the simple reading would be that Adam was a special creation, and the first human being created on this Earth.
But, it is equally true that the conclusion that there were other people on the Earth at the time of the murder that were not Adam's descendents is also the plainest, simplest reading of the text of the Cain story. Again, if read alone, without any other "context" to control the interpretation, NOBODY reading the text would conclude that Cain married his sister, but instead would be positive that there were others not of Adam's descent alive at the time. I think you would have to agree with this statement. Read on its own, that is what you would get.
This means that your interpretation that Cain married his sister is based primarily on what it says earlier in Genesis. It is NOT based on a plain reading of the text on its own, but is based on the text *as informed by other evidence*.
And you know what? This is good. This is exactly what you are supposed to do: you should NOT look at Scripture in isolation of the rest of the evidence at your disposal. You are choosing NOT to take the plain reading of one text because of the evidence found elsewhere which you believe requires a non-plain reading and there is a way you can find to reconcile the two (in this case, the other evidence is the earlier Genesis text which you interpret to read that Adam was the first and only man created and that Eve was the physical mother of all who ever lived).
And guess what, I am with you in this method: I read the Creation accounts and try to interpret them. But I realize that a reading confined to the first word to the last period of those texts might lead me to a false conclusion (just like you believe that limiting yourself to the confined text of the Cain story would lead to error). So, I look at the other evidence available to me to clarify exactly what is meant in those verses, even if it means NOT taking the simplest, plainest reading (just as you do with the Cain story). I look to God's Creation, His other record of what happened, and consider how this clarifies the text for me. Then I look to see if maybe I have been interpreting the text wrongly and if there is another interpretation which squares with this additional evidence. And there is, it all fits.
In particular with the Cain and Adam stories, I do exactly the same as you, just backwards. Rather than read the Adam story and twist around the Cain story to fit my conclusions from the Adam creation account, I read the Cain account as much more plain than the Adam account. So, I start with the idea that there are these other humans around and then go *back* to the Adam creation stories and see if it absolutely requires the conclusion that none others could be living. I conclude that there could, indeed, have been others living.
Personally, I believe that Adam was a special creation, made whole and mature for a special purpose. Although there are a LOT of symbolic possibilities, I don't see any reason not to believe this literally. And, following my prediliction to start with the literal, I read the Cain story in the most literal way: that there were others living at the time.
Now, can both of these be true? Sure, God could have done it a lot of different ways, but didn't bother telling us everything that happened in His creation. Why does God tell us some things and not others? I have no idea.
