But could the reason for that be that Cain wasn't Adam's offspring?
Well sure. You know, it also never mentions in the Bible that many of the prophets were
human beings. Maybe they were actually telepathic aliens.
My point is that you can't always (or even usually) infer the intent of scripture by what is
not said.
Genesis 5:3 is the beginning of a genealogy. Abel apparently didn't live long enough to find a mate and reproduce, but Cain did; yet we never read of him having been begotten by Adam, even in his own genealogy in Genesis 4. And I find the wording of Genesis 5:3 rather striking.
Cain was an outcast. The reason Seth is given prominence is that, with Adam having zero remaining sons (that we know of) who aren't
cursed, Seth represented the plans of God and the future of the human race. In fact, as I recall, we don't really hear about Cain again after that bit about his ancestors. Does this mean that he's some sort of dragon/serpent + human hybrid? Well, I guess possibly, but I think it's just because he was
persona non grata at the family reunions. The reason he's not mentioned in Genesis 5, of course, is because it gives the direct line between Adam and Noah.
Well, the links I gave above certainly believe there was literal forbidden fruit involved, but they argue, from the references to nakedness and birth pains, that it had the effect of making one sexually aware, and this was the reason the serpent wanted Eve to eat it. No motive is given in the text for what the serpent told Eve, yet it says he was the craftiest of all creatures (Genesis 3:1).
Those lnks you gave seem to mostly be a load of bollocks. They infer a lot about Adam and Eve and their beliefs from just a couple lines of ambiguous text. I fail to see how this is anything but a doctrine stemming from Biblical omission. The Bible never mentions that Cain went to the bathroom; therefore, he must be some sort of hybrid. Not going to work.
I feel like the wording of Genesis is so unclear, so shrouded in antiquity, and so myth-like that it's impossible (or nearly so) to infer anything about the creation of man that isn't clearly stated. As I said before, I think that Genesis is more of an allegorical tale. If this is the case, it is completely fruitless (no pun intended) to assume anything about the actual story-behind-the-story, as the veracity of any hypotheses cannot be validated.