Adam and Eve are the parents of all humanity, according to the Bible. That makes all men and women brothers and sisters. You are committing incest because that is your sister you are married to. If you are attracted to a woman, you are guilty of incest for wanting to hump your sister. That's just the way it is.
Update: I see you coonts are still committing incest. Bunch of sister-fucking sods.
Don't take the bible literally, it's a sort of history book. With a lot of added made up stuff in an attempt to keep people on the right path (Therefore they came up with heaven and hell, to scare folks, and as a promise of great rewards if they lead a devout life)