Jesus said this:
[Jesus] said, "It is what comes out of a person that defiles. For it is from within, from the human heart, that evil intentions come: fornication, theft, murder, adultery, avarice, wickedness, deceit, licentiousness, envy, slander, pride, folly. All these evil things come from within, and they defile a person." (NRSV, Mark 7:20-23)
The apostle Paul wrote these verses:
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and 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. (NIV, Galatians 5:19-21)
Flee from sexual immorality. All other sins people commit are outside their bodies, but those who sin sexually sin against their own bodies. Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your bodies. (TNIV, 1 Corinthians 6:18-20)
It is God's will that you should be sanctified: that you should avoid sexual immorality; that each of you should learn to control your own body in a way that is holy and honorable, not in passionate lust like the pagans, who do not know God; (TNIV, 1 Thessalonians 4:3-5)
The Bible does not provide a specific list of acts that constitute "sexual immorality," but these verses, also written by Paul, seem to say that any sexual intercourse, except between husband and wife, would be wrong:
Now concerning the matters about which you wrote: "It is good for a man not to have sexual relations with a woman." But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband. The husband should give to his wife her conjugal rights, and likewise the wife to her husband. For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does. Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer; but then come together again, so that Satan may not tempt you because of your lack of self-control. (English Standard Version, 1 Corinthians 7:1-5)
Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I am. But if they cannot control themselves, they should marry, for it is better to marry than to burn with passion. (NIV, 1 Corinthians 7:8-9)
Related verses: Matthew 15:19-20, 1 Corinthians 5:1-2, 6:9-11, 6:16-20, 7:36-38, Ephesians 5:3-5, Colossians 3:5-6.
However, once married, the religious laws and customs of the time strongly prohibited adultery and provided physical and financial protections for the spouses and their children (Exodus 20:14, Leviticus 18:20, 20:10 Deuteronomy 25:5-6, Proverbs 22:6, Malachi 2:13-16, Matthew 5:32, Mark 10:2-12, Romans 7:2, Hebrews 13:4 Ephesians 5:22-25, 6:4, 1 Timothy 5:8).
In today's world, it is the legal marriage that provides similar protections. Merely having sex or living together does not fulfill the functions of the Biblical model of marriage.
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What Does the Bible Say About Adultery?