A Field Trip To the Creation Museum
I went to the flickr gallery and was stunned and fascinated (shocked and awed?) at the exhibit which "explained" where Cain got his wife and why it was okay for him to marry (and have sex, although the "S" word is never used) his sister.
For your edification I copied out the central "argument" for you to mock (er, I mean discuss.)
"The farther back in history one goes (back towards the Fall of Adam), the less of a problem mutation in the human population would be.
At the time of Adam and Eve's children, there would have been very few mutations in the human genome--thus close relatives could marry, and provided it was one man for one woman (the biblical doctrine of marriage), there was nothing wrong with close relatives marrying in early biblical history."
B.S. (Bedevere Science) all. (SIR BEDEVERE: And that, my liege, is how we know the earth to be banana-shaped.
ARTHUR: This new learning amazes me, Sir Bedevere. Explain again how sheep's bladders may be employed to prevent earthquakes.)