Thou shalt not commit adultery.
What is adultery? Lev 20:10 defines it:
Leviticus 20:10 And the man that committeth adultery with another man's wife,
even he that committeth adultery with his neighbour's wife, the adulterer and the
adulteress shall surely be put to death. [See also, Proverbs 6:32-34, Jeremiah 29:23,
Adultery is defined simply as a married woman lying with a man other than her husband. They are both committing adultery.
Note: Don't confuse fornication with adultery. Fornication is an act with a woman who is not one of your wives or an animal. Fornication is also a sin.