Genesis creation in bible doesn't make sense

I have always been puzzled by the story of creation in the bible. I believe in evolution, but it seems like there is a flaw in the creation story that is so glaring I don't know how the believers could have believed it for 2,000 years, and how they can defend it now.

According to Genesis, God created Adam and Eve who were the first humans. Then Adam & Eve had two sons, Cain and Abel. Unfortunately, Cain killed Abel, so God gave them another son to replace Abel. From this, how did the human race continue? The sons of Adam and Eve never had a woman to breed with, right?

Answer #1 from a liberal priest at my University:
The story should be taken figuratively, not literally.

Answer #2 from a religious grad student:
I've always assumed that Adam and Eve had many children, both girls and boys, and that Cain and Abel are given special mention because of the spiritual lessons they provide us. So, Cain probably married one of his sisters. If so, that probably would not have the harmful genetic consequences that inbreeding does today, because human biology and genetics were fresh from the hand of the Creator, with no "bad" genetic material to pass on. And, for all we know, Cain may not have even grownup with this sister.

However, it seems like creation is a foundation for the entire bible and the Judeo-Christian faith. It seems too important for believers to brush off as just a story not to be taken literally. Also, the answer about breeding with un-mentioned sisters because inbreeding was not a genetic problem then seems like stretching it a lot. If that was the case, the bible should have mentioned it.

Does anyone else have any other answers/explanations to contribute?