Not all sexual covetousness is lust. When a man gives way to lust simpliciter the end result is arousal. When a man gives way to sexual covetousness, the end result is discontent and a vague sense of entitlement.
Here is the distinction. In Romans 7, the apostle Paul says that he would not have known what sin was if the law had not said “do not covet” (Rom. 7:7). The rich young ruler comes to Jesus, and claims to have kept the entire second table of the law, with the one exception that Jesus left out of His list, which was the prohibition of covetousness (Mark 10:19). Now we have to remember that one of the prohibited objects of covetousness was found in the person of your neighbor’s wife (Ex. 20:17).
In Proverbs 5, a young man is instructed, bottom line, to stay out of the wrong beds. He is told to avoid adultery (Prov. 5:20), and he is told this because the Lord is omniscient and sees everything that is going on (Prov. 5:21). The relevance to my point here is found in the preceeding verses, where the husband is told to rejoice with the wife of his youth (Prov. 5:18). He is told to let her breasts satisfy him at all times, and to be ravished, enraptured, delighted, and intoxicated with her love (Prov. 5:19). That’s hard to do when you have been out window shopping for other models. Or supermodels.
The progression toward adultery moves like this — simmering discontent, open discontent, open desire in other directions, which is lust, and then the lust is acted out, with infidelity as the result. Now a man might be able to convince himself that he is not being unfaithful in the first two stages — he is not being aroused, and he is not actively seeking that kind of gratification. His problem doesn’t appear to him to be overtly sexual at all. But it is a set up. Don’t feed the kind of discontent that will, later on, feed something else.
Incidentally, this the trap to avoid when a man has learned what I urged elsewhere in my discussion of nuisance lust. This is the ditch on the other side of the road.