Deuteronomy 27

Verse 22
American Atheists claims the Bible is wrong about the passage (and makes the following comments (italicized).

Incest Deuteronomy 27:22 Cursed be he that lieth with his sister, the daughter of his father, or the daughter of his mother. And all the people shall say, Amen. Leviticus 20:17 And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a wicked thing; and they shall be cut off in the sight of their people: he hath uncovered his sister's nakedness; he shall bear his iniquity. [But what was god’s reaction to Abraham, who married his sister — his father’s daughter?] See Genesis 20:11-12


 * See also Pre-Flood Longevity

As Romans 5:13 says, "sin is not imputed when there is no law." Before the time of Moses there was no necessity for a law against incest because such vast lifespans made for very different family units, and incest was thus not wrong in the same way it is today. The commandment against incest was not given until the time of Moses because incest was not wrong before that, since people lived such extraordinarily long lives, centuries longer than we do today. Technically it wasn't incest by today's standards since early humans each had lifespans of 900 years (Genesis 5).

Incest was only declared wrong by the time of Moses when people had average lifespans of 70 years (Psalms 90:10) and God when the Flood started began reducing the average lifespan to 120 years (Genesis 6:3. Lifespans did not reach this point until about the time of Moses, who lived exactly the 120 years specified. (Deuteronomy 34:7) Incest today is considered wrong, as it should be, because children who grow up with one another as part of the same family should not have sexual relationships. However, ancient people like Noah's family lived centuries upon centuries and could be born many centuries apart in age from their siblings, old enough by today's standards to be the great, great, great, great, great, great, great grandparents of their brothers and sisters.

In the case of Abraham, he lived to be 175 years old, whereas his half-sister and wife Sarah lived to be 127 years old. (Genesis 25:7, Genesis 23:1-2) Both lived longer lives than anyone today can live. People lived such long lives at the time that Abraham's great, great, great, great, great, great, great, great grandfather, Noah himself, did not die until 18 years after Abraham was born, at the ripe age of 950 years old! Under the Biblical model, such ancient patriarchs could have siblings considerably older and more distant in relation than is seen today, which is why the commandment against incest was not yet given, and incest at the time was not wrong.