Christians...

[Razzle]

Where in the Bible is incest described as a sin? Isn't it just one of those things Christians assume is in there because it matches their own views, like only "one man, one woman" marriages being acceptable despite the rampant condoned polygamy and concubines, and God considering the killing of humans in utero to be the crime of murder when the punishment was a mere fine in Old Testament law?

Read this http://www.gotquestions.org/incest-in-the-Bible.html it explains it. 

Great, thanks! A few interesting things in there:

The genetic explanation that page gives for incest being wrong after Leviticus and not before it, doesn't explain some of the rules in the Leviticus passage they cite (Leviticus 18:6-18), i.e. " You shall not uncover the nakedness of your father’s wife’s daughter, brought up in your father’s family, since she is your sister" and "You shall not uncover the nakedness of your father’s brother, that is, you shall not approach his wife; she is your aunt."

Also, these seem to be rules regulating, and therefore continuing to permit, polygamy: "You shall not uncover the nakedness of a woman and of her daughter, and you shall not take her son’s daughter or her daughter’s daughter to uncover her nakedness; they are relatives; it is depravity. And you shall not take a woman as a rival wife to her sister, uncovering her nakedness while her sister is still alive."

[Spooky]

If we assume the story of creation is literally true (let's assume that for a second so don't disagree) and believe Adam and Eve procreated and had their sons Caim and Abel - And then were expelled from paradise - So... Since there is no other human being to procreate with, that means we all inevitably descend from incest. 

My question is - Why is incest a sin if we all come from incest?

Because god used of all of her mojo for the first few generations to keep us from having 11 toes and eyes too close together that now it doesn't matter how hot our sister is?

[Wyrd of Gawd]

If we assume the story of creation is literally true (let's assume that for a second so don't disagree) and believe Adam and Eve procreated and had their sons Caim and Abel - And then were expelled from paradise - So... Since there is no other human being to procreate with, that means we all inevitably descend from incest. 

My question is - Why is incest a sin if we all come from incest?

If you read the Adam & Eve story in Genesis chapter 2 you will see that the Garden of Eden was just down the road from Assyria, Persia, Arabia, and Ethiopia.  The region was crawling with people.  The main characters in the Bible are descended from Adam & Eve ending with the Jesus character as the final one.  Adam & Eve have zero to do with the rest of humanity.  

In the Old Testament most of the main characters are related through incest.  Abraham screwed his sister till the day she croaked.  The bigot Rebekah went ape shit when Esau refused to marry his relatives and took foreign wives.  Lot's skanky daughters got knocked up by him despite the fact that there were people all around them.  There are numerous other stories along the same line.  

The Israelites/Jews didn't want to mix with other people because they didn't want to lose their property to strangers.  They wanted to keep it all in the family through incestuous marriages.  That's why the younger brother had to marry the older brother's wife when the older brother died.