Incest not necessarily a problem for man nor beast.
Let's say the bible is myth. The stories presuppose the existence (in the book) of a being who can do miracles - like creating all these things from scratch. That puts him in charge of their genetics. Additionally, it's one of many such books describing a past Golden Age, in this case one of human perfection. That didn't include walking on water or teleporting, but did include a conditional deathlessness. (Gen 2:15-17)
In order for normal reproduction to carry out the First Mandate (Gen 1:28), that perfection would carry with it either (a) immunity from inbreeding problems or (b) the guarantee of divine intervention a/r. Since, back in the real world, incest does not guarantee genetic defects (although I agree it's not a good thing), choice (a) has logic to it.
All of the above is consistent also with the 'unbelieveable' life spans recorded in the same book. (And, according to R. Dawkins and his kind, how long should we live if science is allowed to assist evolution?)
(Sidebar: The account of Jacob's dealings with his father-in-law includes an example of (b). The account at Gen 30 implies that Jacob thought Lamarckist genetics would help him. That certainly would be the case with a fictional character, or his mythmaker, who didn't know what we know. But the end of the story shows the origin of both Jacob's actions and their success in a way that's internally consistent. (Gen 31:5-13) And it's consistent with the professions of belief of many scientists - too sensible to believe in myths - who are sure they will soon be engineering a brave new world of lifeforms.) 
Anyway, at least it's a pleasant myth.
(Rev 21:3,4)