The Sixth Commandment

Jesus and the woman taken in adultery.

You shall not commit adultery.

This, at last, is straightforward. You must not have sex with someone if either of you is married, and not to each other.

Although it has been extended in both Christianity and Judaism to refer to any sex outside marriage, strictly, it is this that is prohibited.

And the reason seems plain: violation of trust. Betrayal. Marriage is a promise of sexual exclusiveness.