Please show where scripture teaches that Cain and Able were their "only" children.
Adam lived 930 years. And just so you don't embarrass yourself.
Genesis 5:4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:
830 years is a lot of time to have sons and daughters.
I am a little embarrassed since I forgot to mention Seth ,and Adam's further offspring in Chapter 5 which included daughters. So then Cain lived long enough (100's of years) to wed one of those sisters? Is that how it worked?
Also, in Chapter 4: "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."
Who was there to slay Cain before he found his wife and begat his offspring and before Seth's offspring? Do I have the timeline correct?
With due respect to your faith, I stand by my belief that the OT is mostly fable. BTW, who wrote Genesis? Serious question.