Exodus 20:14  
Thou shalt not commit adultery.

Leviticus 20:10 (NIV)
"`If a man commits adultery with another man's wife—with the wife of his neighbor—both the adulterer and the adulteress must be put to death.'"

Deuteronomy 22
²² If a man is found sleeping with another man's wife, both the man who slept with her and the woman must die. You must purge the evil from Israel. ²³ If a man happens to meet in a town a virgin pledged to be married and he sleeps with her, ²⁴ you shall take both of them to the gate of that town and stone them to death-the girl because she was in a town and did not scream for help, and the man because he violated another man's wife. You must purge the evil from among you.

Proverbs 6:32 (NIV)
But a man who commits adultery lacks judgment; whoever does so destroys himself.

Matthew 5
Adultery
²⁷"You have heard that it was said, 'Do not commit adultery.' ²⁸But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. ²⁹If your right eye causes you to sin, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. ³⁰And if your right hand causes you to sin, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to go into hell.

Divorce
³¹"It has been said, 'Anyone who divorces his wife must give her a certificate of divorce.'  ³²But I tell you that anyone who divorces his wife, except for marital unfaithfulness, causes her to become an adulteress, and anyone who marries the divorced woman commits adultery.

Galatians 5
¹³You, my brothers, were called to be free. But do not use your freedom to indulge the sinful nature; rather, serve one another in love. ¹⁴The entire law is summed up in a single command: "Love your neighbor as yourself." ¹⁵If you keep on biting and devouring each other, watch out or you will be destroyed by each other.

Life by the Spirit
¹⁶So I say, live by the Spirit, and you will not gratify the desires of the sinful nature. ¹⁷For the sinful nature desires what is contrary to the Spirit, and the Spirit what is contrary to the sinful nature. They are in conflict with each other, so that you do not do what you want. ¹⁸But if you are led by the Spirit, you are not under law.
¹⁹The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; ²⁰idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions ²¹and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God.  ²²But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, ²³gentleness and self-control. Against such things there is no law. ²⁴Those who belong to Christ Jesus have crucified the sinful nature with its passions and desires. ²⁵Since we live by the Spirit, let us keep in step with the Spirit. ²⁶Let us not become conceited, provoking and envying each other.

Luke 16:18
Whosoever putteth away his wife, and marrieth another, committeth adultery: and whosoever marrieth her that is put away from her husband committeth adultery.

Mark 10:12
And if a woman shall put away her husband, and be married to another, she committeth adultery.

1 Corinthians 6:9, 10
Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind ... shall inherit the kingdom of God. 

Hebrews 13:4
Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge.

Matthew 15:19
For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies....

Proverbs 28:13
He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy. 

1 John 1:9
If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.

John 8:11
... Jesus said unto her, Neither do I condemn thee: go, and sin no more. 

Ephesians 4
Living as Children of Light
¹⁷So I tell you this, and insist on it in the Lord, that you must no longer live as the Gentiles do, in the futility of their thinking.

Ephesians 5
³But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people.