Similar to murder, Jesus addresses not just the sin of adultery but also the internal process behind the act of adultery. It is unlikely many people in his original audience were serial adulterers and no one would have considered adultery to a positive influence on society.
The source of the problem of adultery seems to be “looking where one ought not look,” a point made in the Second Temple wisdom book Sirach. Sirach is instructing young men and he is certainly not politically correct from a modern perspective. (If his words offend, try switching the pronouns, instead of “Turn away your eyes from a shapely woman” change it to “Turn away your eyes from a shapely man.” It works either way.)
Sirach 9:1–9 (NRSV) Do not be jealous of the wife of your bosom, or you will teach her an evil lesson to your own hurt. 2 Do not give yourself to a woman and let her trample down your strength. 3 Do not go near a loose woman, or you will fall into her snares. 4 Do not dally with a singing girl, or you will be caught by her tricks. 5 Do not look intently at a virgin, or you may stumble and incur penalties for her. 6 Do not give yourself to prostitutes, or you may lose your inheritance. 7 Do not look around in the streets of a city, or wander about in its deserted sections. 8 Turn away your eyes from a shapely woman, and do not gaze at beauty belonging to another; many have been seduced by a woman’s beauty, and by it passion is kindled like a fire. 9 Never dine with another man’s wife, or revel with her at wine; or your heart may turn aside to her, and in blood you may be plunged into destruction.
Just like the previous expansion of “do not murder” to include the mental processes behind murder, Jesus points to the internal thoughts and attitudes which lead to adultery. Rather than “looking where one ought not,” perhaps it should be “thinking what one ought not.” I had a friend in high school who was told by his parents it was okay to look, but not touch. This led to a series of seriously bad choices for him over the years. Jesus is clear: it is impossible to separate thoughts from actions. Internal anger will come out as rage and destructive words. Lust will develop into some external behavior.
For most younger people in the west, sexual attitudes are formed online where pornography is easily discovered. The worldview of pornography is damaging to both men and women and skews a biblical view of sexual relationships and marriage. The Bible celebrates sexual relationships (although that does seem to be the case for some Christian teaching).
What goes on inside someone’s head is impossible to see, we tend to think that no one knows or cares that we have thought impure thoughts. Jesus explodes this by comparing those private thoughts to the act of adultery itself. As with Jesus’s teaching on anger, it is important to at least observe he is teaching all his disciples to control their internal lust, both men and women. Although men are usually the problem, female disciples of Jesus are called to the same high standards as the men.
Jesus is saying, in effect, “don’t let your eyes make you sin.” Don’t put yourself in a position to look lustfully. Jesus often uses hyperbole to shock his audience, to pluck out an eye is an exaggeration since a blind man can still lust. The prime example of this is David, who saw Bathsheba and then committed adultery. Should he have “plucked” his eye out? No, but he should have had the sense not to be in that position to see Bathsheba in the first place.
The problem for the modern reader is how to draw implications of Jesus’s teaching to new situations. As I write this, the #MeToo movement is still developing and the alleged immoral behavior of a Supreme Court candidate is in the headlines.
Is it narrow-minded to apply Jesus’s words to the epidemic of sexual harassment women have faced for generations? A commitment to marital fidelity often results in people calling you a prude, a Puritan, etc. But if Jesus was correct about internal anger, is he also correct about the dangerous effects of internal lustful thoughts?