Question: "Why is 'You shall not commit adultery' in the Ten Commandments?"
Answer: Before we can answer this question, we need to be clear on the definition of adultery. The dictionary defines “adultery” as “voluntary sexual intercourse between a married person and a person who is not his or her spouse.” The Bible would concur with this definition. In Leviticus 18:20, God told Moses, “Do not have sexual relations with your neighbor's wife and defile yourself with her,” and in Deuteronomy 22:22, we find a similar definition: “If a man is found sleeping with another man's wife, both the man who slept with her and the woman must die.” It is clear from these definitions that adultery refers to a consensual sexual union. What is not explicitly clear is whether or not both partners in this illicit sexual union are married. The biblical commands prohibit a man from taking another man’s wife, but do not indicate whether or not the man is also married. Be that as it may, it is safe to say that if a person who is married willingly seeks a sexual encounter with another person, whether or not that person is also married, both people are guilty of committing adultery.
God’s reasons for instituting His commandment against adultery are two-fold. First, God established the institution of marriage as being between one man and one woman (Genesis 2:24; reiterated by Jesus in Matthew 19:5 and parallel passages). God created marriage to be the building block of His creation and of society. Even after the fall (Genesis 3), marriage is still a sacred union and the foundation for society. In marriage, the full expression of the image of God is made manifest as the man and the woman complement and complete each other. The Bible also teaches us that marriage is the vehicle through which God designed the procreation of the human race and the preservation of godly offspring (Genesis 1:28, 9:1; Malachi 2:15). With such a premium placed on marriage, it’s no wonder God would seek to protect this union from defilement (Hebrews 13:4), and thus prohibit adultery, which is the violation of the sacred marriage union.
The second reason for the commandment is found in Leviticus 18:1-5. As God’s chosen people, the Israelites were to reflect God’s character in the Promised Land. God commanded His people to be holy for He is holy (Leviticus 11:44), and part of holy living is sexual purity. God did not want His people emulating the behavior of the Egyptians from whom He delivered them, nor did God want His people copying the behavior of the people into whose land He was bringing them. The implication was that adultery (and other sexual sins) was commonplace in the lands where the Israelites had been and were going to.
So now we know what adultery is and why God instituted this command. Finally, we need to learn what God meant by the command itself. As with all of the Ten Commandments, there are things we need to avoid doing (the negative part of the command) and things we need to be doing (the positive part of the command). The negative part of the command is self-explanatory: Do not commit adultery. However, there is more to this command than the simple avoidance of extramarital relationships. One can make the argument that wrapped up in this prohibition are all sorts of sexual sin (e.g., incest, fornication, homosexuality, etc.), and that argument can be made on the basis of chapters such as Leviticus 18. Also important is avoiding things that would lead or tempt one to consider adultery, such as the unnecessary withholding of conjugal rights (1 Corinthians 7:1-5). Jesus, in His Sermon on the Mount, made further clarification of this command (Matthew 5:27-30) by including all kinds of lustful thoughts. Fantasizing about having sexual relations with someone is the same, in God’s eyes, as actually committing adultery. Therefore, we must avoid all things that would create within us lustful thoughts (e.g., suggestive songs, sensuous movies, pornography, etc.). We should also avoid immodest clothing or anything that might cause a brother or sister in the Lord to stumble in this area (1 Timothy 2:9; 1 Peter 3:3).
The positive part of the command would entail doing the opposite of what the command prohibits: chastity in body, mind, words and action; keeping watch over what we take in with our eyes and the other senses; an attitude of temperance and self-control (i.e., moderation); being discerning over the company we keep; dressing modestly; and fulfilling our marriage vows in regards to sexual relations and cohabitation. Regarding sexual sin, the Apostle Paul said, “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body” (1 Corinthians 6:18). When it comes to sexual sin, the best course of action is to remove ourselves from temptation and avoid such situations altogether.
Adultery is the complete corruption of God’s good creation of marriage. Through the sin of adultery, Satan tempts us to seek sexual fulfillment in avenues other than the one God has ordained—within the bounds of monogamous, heterosexual marriage. Adultery rips at the fabric of society because it tears apart marriages and families which are the building blocks of society. God’s law in general, and the 7th commandment in particular, is held up as the standard for Christian behavior.