Cain probably did marry his sister.
From a creationist standpoint, Adam and Eve were perfect genetically, and their children could reproduce without side effects. This could continue for quite some time, before the problem of inbreading would result in problems, as it does today. So genetically we are devolving.
This fits much better than the belief that we were genetically lessors, and through obvious inbreeding for thousands of generations, we evolved forward, while our propensity for problems from inbreeding increased.
So meiosis didn't start until post-Eden? How were the gametes produced in Adam and Eve's bodies before then?
This could not continue for some time because each time meiosis happens there is independent assortment and this is where genetic variability comes from. Without meiosis, we would all look the same and there would be none of those "mistakes" mutations that lead to new phenotypes that are thrown out into the world. Some of those phenotypes help organisms to live longer and thus those phenotypes get naturally selected.
One example of this is sickle cell anemia, it has been around for a long time. Shouldn't it have died out with its victims? Why has it seemed to be naturally selected to continue? The interesting thing is that where sickle cell anemia is prevalent, malaria is the number 1 disease. And it just so happens that those with the sickle cell anemia recessive gene are curiously immune to malaria (the more fatal disease).
"So genetically we are devolving"Biologically, that makes no sense. Evolution is simply change over time, you can't devolve. You either change (evolve) or you do not. And not evolving in some way is not possible if you reproduce.
And we were genetic lessors in some ways, and better in others. See the bubonic plague, those that survived it many years ago were immune to it and thus they reproduced and their offspring were passed that BP immunity allele. That is one main reason you don't see many cases of those deadly bacteria infecting anyone.
Every time a human is produced, many many things can go wrong genetically. More goes wrong when you inbreed with close relatives because the odds of two relatives sending two recessive genes is higher. This is because of the allele similarities among family members.