1. As ya'll know, the broader, more important context is about going further and deeper than mere outward adherence to the letter of the law.Jesus - Matthew 5:27-28 (ESB) - wrote: 27 You have heard that it was said, Do not commit adultery. 28 But I tell you, everyone who looks at a woman to lust for her has already committed adultery with her in his heart.
2. In verses 27-28, he uses the sin of adultery to again make that point. (Intentionally desiring a sin is as much a sin as actually carrying it out.)
3. Jesus is probably not intending here to redefine the word "adultery".
4. Insofar as these verses give us instruction about sexual behavior, it can really only responsibly be used to prohibit adultery and the desire to commit adultery, which is of course an important prohibition throughout both testaments.
5. It is obviously not necessarily wrong to set one's heart upon (long for, desire, covet) a man or woman. These verses only really deal with the sort of intentional desire that if actually carried out would result in an adulterous affair.
Nothing new or profound, I know, but I think that many teachers don't want to get into this lest they be accused of encouraging or excusing some type of immorality, and I find that interesting. So, the passage is often misapplied, I think, though with good intentions, no doubt. What do you think?