In a question and answer forum I was asked by a student how Adam and Eve's children could have had children without incest. I gave a two-part reply. They appreciated the last part the best.
I replied that they didn't. Since there were no other humans, Adam and Eve's children would have had no one else to marry but each other. But, this was before incest was specifically forbidden and it wouldn't have been sin. And as far as deformities caused by incest, I think it is safe to presume that there must have been something genetically pure or different about the early humans. In Genesis we read about early humans living extremely long lives. Also, Adam and Eve's DNA would have contained the diversity of all the races. So there was probably something special about their genetics that kept everyone from being born with arms growing out of their heads.
But if someone thinks to themselves that this is a far-fetched and stupid and that evolution makes more sense, then think about this: If Adam and Eve's kids had a problem, what kind of problem would there be for evolution? When the first monkey evolves into a human, there is just one human being. Who does that guy have to marry? A monkey. At least with Adam and Eve there were two human beings to get things started. With evolution, the first human being has to hook up with an animal. So you tell me which is more messed up?
Yes, I know that no one thinks that monkeys evolved directly into human beings. I was obviously being rhetorical. Still, the inescapable fact is that if evolution is true, then the first human had no one to mate with except for a non-human.