The son of Cain named Enoch is not the same Enoch referenced in Genesis 5:24.
But really the point is the word of the LORD is the same yesterday, today and in perpetuity so the possibility that incest would have been necessary or required in order for mankind to populate the earth is naught, and the probability nil. In other words, if it was permissible then it would be permissible now.
However, I got to give credit to Richard Dawkins for acknowledging the commonality between the accounts of man's origin and thus proposes that mankind didn't evolved from one set of two progenitors but one set of three progenitors in a lecture entitled "
There never was a first person."
Those three being the Y-Chromosome Adam, the Mitochondrial Eve and the third being our most recent common ancestor. However, many don't recognize where that concept originated from but then they wouldn't believe a parable anyway.