What does the Bible teach about adultery (extramarital sex) and why?
The word adultery means, according to the Merriam-Webster online dictionary, "voluntary sexual intercourse between a married man and someone other than his wife or between a married woman and someone other than her husband." In other words, adultery occurs when one or two married people have sex with someone they are not married to.
God included a command against adultery in the Ten Commandments: "You shall not commit adultery" (Exodus:20:14). "Since God's law sanctions sexual relationships only within a legitimate marriage, the command to not commit adultery covers, in principle, all varieties of sexual immorality".
Why does God hate adultery? Because He places heavy emphasis on the sanctity of marriage, and He knows adultery is a terribly destructive attack on marriage. God uses the marriage relationship as a metaphor to explain Christ's love for His Church (Ephesians:5:25). When a husband or wife commits adultery, it shatters (sometimes irreparably) the bond that the two share and fills the relationship with distrust and bitterness.
God also uses adultery as a metaphor for when we as Christians turn our backs on Him (see Jeremiah:3:8). When we put other priorities before God after we've dedicated ourselves to Him, we, too, damage the bond between us. However, unlike humans, God is always willing to welcome us back after sincere repentance on our part (Isaiah:1:18).
Still, adultery is a sin that causes incredible physical, emotional and even spiritual damage. We should take great care to avoid it, whether in our human or spiritual relationships.
For more information, please read our booklet Marriage and Family: The Missing Dimension .