The focus in this case was about explaining that not only was outward behavior sinful, but the inner thoughts, as well. It was a knock against the allegedly “pious” who believed themselves sinless. Lust is also different from appreciation. The notion, here, isn’t simply attraction, which is passive; it is desire. It’s ἐπιθυμῆσαι, with an active voice, meaning there is an intent and decision to engage mentally with the attraction felt. It’s the difference between thinking the neighbor’s steak smells good on the grill as opposed to craving it, imagining what it tastes like and how you might acquire some. When both are eligible, it is understandable to view a woman appraisingly, but you are expected to exercise caution when you do so, unless it might move beyond appraisal and appreciation into desire. Further, this action should be reserved for single men approaching single women, not married men toward any woman, or any man toward married women. Hence the use of the word ἐμοίχευσεν for the act of “committing adultery.” The word ἐπιθυμῆσαι is related to words sometimes translated as “covet,” much like the wanting another man’s car for yourself. Additionally, the word can have positive connotations when about the “desire” to do what is right (Matt 13:17 re: prophets and righteous men who wished to see Christ incarnate, but died before his arrival). The word, while sometimes used in proximity, is also different from πορνείας (Rev. 2:21), specifically meaning “sexual immorality,” or any thought or action of a non-monogamous sexual nature.