“You shall not commit adultery.”
G.K. Chesterton said, “If men will not be governed by the Ten Commandments, they shall be governed by the ten thousand commandments.” The point is that in the brevity of these commandments, both in number and in the words used, God is acting toward us in mercy. He could have easily overwhelmed us with commands, exceptions, and rules upon rules (which is what the Pharisees did), but he did not do this. He gave us ten simple commands like this one: Don’t commit adultery. Its brevity is its mercy; its universality is its severity.
Scripture presents God’s ideal of sexual union between husband and wife as an exclusive relationship. Neither the man nor the woman may be sexually active with anyone else, for the marriage bed is to be honored and kept holy. While other societies allowed for married men (but not women) to sleep around with other people (who were themselves unmarried), the God of Israel nowhere makes such allowances. The people of God, both men and women, are prohibited from all manner of sexual license. Sex is the union of a man and his wife. Everything else is considered sin.