This is the thirteenth post on Christian ethics in my ongoing series, theology in bullet points. The first unit in this series had to do with God and Creation (book here), and the second unit was on Christology and Atonement.
We are now in the third and final unit: The Holy Spirit and the Church. The first set of posts in this final unit was on the Holy Spirit. The second set was on the Church. The third set was on sacraments. This final section is on Christian ethics.
Thou shalt not commit adultery.
1. The Bible unanimously rejects adultery, making it a point of Christian ethics on which there is little disagreement. Today, we would define adultery as a person being unfaithful to his or her spouse by having sex with someone who is not his or her spouse.
In Old Testament times, the function of laws against adultery was surely to preserve social stability. In the patriarchal culture of the Ancient Near East, it was activity against the man that was primarily in view. Bruce Malina has suggested that the default definition of adultery in biblical times was "the action of dishonoring a male of one's community by having sexual relations with his wife."  In an honor-shame world, to shame a man by sleeping with his wife easily led to feuds and violence within the clan. It was therefore strictly off limits.
The adultery prohibition of the Old Testament, therefore, was heavily oriented around the man. If Malina is correct, then adultery was a wrong done to a man. By definition it was not committed against a woman. It was the later biblical prohibitions on divorce that served that purpose--the purpose of protecting the wife. In the earliest understandings, therefore, a man by definition never technically committed adultery against his wife. He committed adultery against the husband of any wife with whom he slept.
There were, however, few ways for a man to have sex except with his wife. He could not have sex with another man's wife, for that was adultery against that man. He could not have sex with an unmarried, unengaged woman without penalty (e.g., Exod. 22:17; Deut. 22:28-29).  Normally if he had sex with such a girl, he would need to take her as an additional wife (Exod. 22:16), remembering that polygamy was a normal practice in the Old Testament world (e.g., Deut. 21:15).
In theory, a man might visit a prostitute without committing adultery (e.g., Judah with Tamar), but it was against the law for Israelite man or woman to be a prostitute (Deut. 23:17). It was also against the law for a man to have sex with another man (Lev. 20:13). In short, although adultery was probably defined as a wrong against a man, there were tight restrictions on ways in which a man might have sex with someone other than his wife.
2. Jesus completely refocuses this equation. Since his ethic is filtered through the love command, we now come to understand adultery as much in terms of harm as in terms of shame. Adultery and divorce are wrong because they harm a husband or wife. In one of Jesus' most ironic statements, he suggests that a man who divorces his wife causes her to commit adultery against himself (Matt. 5:32). By in effect forcing her to marry another man, he causes her to bring shame on himself! 
It is thus completely appropriate that Christians today understand adultery as a wrong that can be committed against husband or wife. The husband wrongs his wife if he has sex outside of the marriage, and the wife wrongs her husband if she has sex outside of the marriage. The degree of wrong is the same, whoever the offending party is. You cannot love your neighbor as yourself and betray him or her in this most sensitive domain of human existence.
3. Indeed, as Jesus refocuses ethics as a matter of the heart, he takes the offence down to the level of intention. In effect, a person can commit adultery without actually committing the act itself. "Everyone who looks at a woman with lust has already committed adultery with her in his heart" (Matt. 5:28). A person can thus commit adultery in one's mind, being unfaithful to his or her spouse.
Perhaps it is important to say that Jesus is not talking about temptation, which in itself is not sin (e.g., Jas. 1:13-15). He is not talking about passing thoughts. The ancient world was not an introspective world, as ours is. He is talking about plotting and fantasizing. Morality in this area involves intent.
Nevertheless, Jesus' ethic extends the question of adultery into the domain of emotional affairs. One can be unfaithful to one's spouse without engaging in sex. One can be unfaithful to one's spouse by allowing an unhealthy emotional attachment with a sexual dimension to develop between yourself and someone else.
4. It is thus important for Christians to have healthy boundaries between themselves and others toward which they might develop a sexual attraction. Some common sense examples might be to avoid being alone with such individuals and avoiding topics of conversation that are prone to create emotional intimacy.
Even more healthy is not to view this issue so much in prohibitive as positive terms. We should love our spouses with the fullness of our sexuality and seek for them to be the focus of our human intimacy. We can and should be close to other individuals with whom there is no sexual dimension, but our spouse should be the focus of our relational dimension, along with the rest of our family.
Marriage is a covenant, a life-long commitment of exclusivity made before God and society. It should not be entered into lightly. It should not be dissolved except under the most unusual circumstances. And it should not be betrayed by shaming ourselves with someone else.
Thou shalt not commit adultery.
ET14: Divorce is usually hostile to God's fundamental values.
 Bruce J. Malina, Social-Science Commentary on the Synoptic Gospels (Minneapolis: Augsburg Fortress, 2003) 45.
 If a man had sex with a woman promised to another, it was treated more or less the same as adultery (Deut. 22:23-27).
 So also Malina, Social-Science Commentary, 46.