For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
J. Mosley
An interesting discussion of incest, genetic purity and related issues can be found in the Doorway Papers by Arthur Custance. Follow this link to a discussion of The Geneologies of the Old Testament wherein he addresses the issue of incest, i.e., marrying a close relative.
http://www.custance.org/Library/Volume7/Part_V/Chapter1.html
Mark Neidert
Are you are suggesting/stating that the fall affected us at even the genetic level and that as time progresses, we are getting worse at the very building blocks of our existence? Presumably, if this is true, then everything else is getting worse too. In effect, each generation would grow more and more wicked. Their bodies more corrupt, their thinking more corrupt, the planet they live on more corrupt.
Is that what you are saying?
I had considered that the fall affected everything (people, planets, solar systems, etc) but I had not even thought that things are getting progressively worse.
If that is what you are saying. I may have it wrong.
Danny Hickman
God didn't "allow incest in the bible."
Allow ~ to give permission to or for.
Where "in the bible" did anyone ask God for permission to have sex with a family member, or (while we're on a similar subject matter) permission to marry more than one person? The answer is it isn't recorded in scripture where anyone asked God for permission to do these things.
God did not "allow" sin. If marrying more than one person, or having sex with close family members is not what God has in mind for us, it was never what God had in mind for us. God doesn't change (Mal 3:6).
I think the person asking this is trying to get an idea of why God didn't stop people from having sex with close family members. My answer is that God wanted us to be eternally tied to each other. We all come from one couple, two people. But God wanted us to expand the species after we'd gotten thoroughly established as a species. Intermarrying was done by people who were born in sin. Not Intermarrying wouldn't solve anything. When the time was right, God put a stop to it. He did the same thing in the case of polygamy.
Why does God "allow" sin? He doesn't ! To allow is to give permission to or for something. A better question is why doesn't He put a stop to sin. He intends to: "The Son of Man will send out His angels, and they will gather out of His kingdom all things that offend, and those who practice lawlessness.." (Mt 13:41).
God has never "allowed" sin, and He plans to end it.