Proverbs 6:32: Whoever commits adultery with a woman lacks understanding; He who does so destroys his own soul.
Galatians 5:19-21: Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
Mark 10:11: So He said to them, “Whoever divorces his wife and marries another commits adultery against her.
Leviticus 18:20: Do not have sexual relations with your neighbor’s wife and defile yourself with her.
Matthew 5:27: You have heard that it was said, ‘Do not commit adultery.’ But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart.
And if that is too many words for your simple mind, here:
Exodus 20:14: Thou shalt not commit adultery.