For additional notes and resources check out Douglas’ website.
So far
We have studied four commandments related to God and 2 related to our fellow human beings. Here we find ourselves in the middle of the ethical commandments.
Sexual ethics is an area considered to be highly personal and "private," yet through most of human history sexual mores were never a matter of personal preference.
Of all the commandments, this one may generate the greatest push-back. Western culture seems obsessed with breaking the 7th and 10th commandments.
Text
The 7th commandment
Exodus 20:14: You shall not commit adultery.
Comments
"The Adulterous Bible" of 1631 accidentally omitted the not (oops!). Watching movies and popular TV sitcoms, one would think God's will for us did include extramarital affairs!
Like the prohibitions on murder, theft, and lying, this prohibition is universal. All religions condemn it. Only oversexed and irreligious (modern western) culture celebrates adultery.
Whereas in the ancient world men often had wives (for procreation) and mistresses (for recreation), in God's word we are to be one-woman men, or one-man women (to use Paul's phrasing in 1 Tim 3 and Titus 1). Such dedication outside God's people is as rare today as it was back then.
Adultery was originally a property crime, a sin against the husband. Of course it is more. In Psalm 51, we read that David's sin (2 Samuel 11) was fundamentally against God, even though he's sinned against the woman he lusted for, the husband he displaced, and the accomplices to his adultery (his servants and Joab). Like all sin, sexual sin is an offense against God. In fact, failure to obey the 5th - 10th commandments is always a sin against God.
Jesus ups the stakes, doesn't he (Matthew 5)?
Not just heterosexual sin; homosexual too. If homosexuals were truly married, then they would be forbidden to cheat, yet this is hardly the norm. It is not unfair to state that they are unfaithful to God, to God's plan for sexual fulfillment, and to each other.
Yet the most widespread sexual sin isn't homosexual, but heterosexual. And you don't have to be married, or be romantically involved with a married person, to commit sexual sin.
Application
Adultery is a discrete action, though like murder it may also take place in the heart. With this is mind, do I allow my eyes or thoughts to wander? Do I avoid places in the city or on the web where I am likely to be tempted?
Do I have a sufficiently intense spiritual life -- am I busy? -- that I don't wander into sexual sin (2 Samuel 11)?
Do I take steps to keep my heart pure by spending time daily in God's word and prayer?
In the ancient world, one could expand his sexual options without technically sinning (take another wife). In the modern world, there are many ways to sin sexually without technically violating another individual. Do I rationalize such behaviors?
If I'm married, do I have eyes only for my husband or wife (Proverbs 5, Job 31)?
Have I bought into the devil's lie that our private life has little effect on our public competence? (E.g., how can we trust politicians who are unfaithful to their own wives not just to follow self-interest instead of honoring their office?)
In the Bible, spiritual unfaithfulness is frequently compared to adultery (e.g. in Hosea and in James 4). Is there any way in which I am "two-timing" on God?
Can we pray Psalm 19:12-14 with sincerity?
God's holiness is for our well-being, health, and happiness (Psalm 119:32).
Further study
Take a look at some of the Koine Greek words for various sins here.
Learn about the destructive effects of cohabitation.