Leviticus 18:6 says, “None of you shall approach to any that is near of kin to him, to uncover their nakedness: I am the LORD.”  From verse 6 through verse 18, the Lord is giving instruction to His people concerning uncovering the nakedness of certain ones. 

To ‘uncover someone’s nakedness’ is referring to having sexual relationships with someone.  Let us read this same verse in the NABRE version.  It says, “None of you shall approach a close relative to have sexual intercourse. I am the Lord.”  The GNT translates this verse as, “The Lord gave the following regulations. Do not have sexual intercourse with any of your relatives.”  In verses 6-18, many close relatives are mentioned.  A person’s mother, father, step-mother, sister, niece, and so forth. 

As you read down the list of those of whom a man must not “uncover their nakedness”, or have sex with, you will probably think that these acts would have been strictly forbidden already.  Who among us would not be disgusted by the very thought of having sexual relations with such close relatives? 

However, there were times when this behavior was not strictly forbidden.  Indeed, it was allowed. 

When we consider how Cain found a wife, we can only determine that he married his sister or possibly a niece.  Since Adam and Eve were the only ones created by God’s creative hands, everyone since was born by a natural birth.  Cain would have naturally had to marry a near relative, there was no one else but those in his immediate family. Genesis 4:16-17 says, “And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.”

We later read of Abraham marrying his half-sister.  Speaking of Sarah, his wife, Abraham said, “And yet indeed SHE IS MY SISTER; she is the daughter of my father, but not the daughter of my mother; and she became my wife.” (Genesis 20:12).  We also read in Exodus 6:20, “And Amram took him Jochebed HIS FATHER’S SISTER to wife; and she bare him Aaron and Moses: and the years of the life of Amram were an hundred and thirty and seven years.”  Here, Amram married his father’s sister…his aunt.

Why were such relations tolerated in the Bible?  I can think of two possibilities.  Since the people were few, the Lord permitted these relationships to allow population growth.  One reason that incest is forbidden in the world today is the realization that reproduction between close relatives has a much higher risk of causing genetic abnormalities. In the early days of the human race, though, this was not a risk due to the fact that the human genetic code was relatively free of defects.

It was only when the Lord instituted these limits in the law of Moses that this behavior was strictly forbidden and is still forbidden by our laws and common decency.  (464.6)