Rick: Noticing that someone is "hot", or beautiful, or even sexy, doesn't necessarily mean that one is lusting after that person.
Exactly! It is God who made men visual and appreciative of the female attractions and form. A man can appreciate these and feminine beauty without sinful lust.
What makes lust SINFUL, however, is going beyond attraction or appreciation of a woman's beauty to fantasizing or sexually desiring a woman other than one's spouse. Why is such desire SINFUL lust (greatly desire)? It's whenever one is sinfully desiring someone who is not theirs! You can't SINFULLY lust for your wife or your husband - lust for one's spouse (sexual desire) is normal and God-ordained. But anyone you are not married to, to sexually desire them is sinful lust. Lust is merely a passionate desire for a person or thing. And when it comes to a person, only lust for what is yours is okay - all other lust is sinful!
All that said, we have to be careful about what is appropriate to look at. One could look at nude photos of people not one's spouse - which isn't a view to see or contemplate, as 1) this view is not for you (it's not your spouse), and 2) intense focus upon body parts one see's as sexual can easily turn to lust. I'd also say that what is or isn't appropriate for a person to view of a naked person can be somewhat cultural. Ever see those National Geographic Magazine photos of tribesmen surrounded by topless women. Notice how bored the men in those photos typically look? It's because, since childhood, seeing women bare-breasted is so normal that they don't associate naked female breasts in they hyper-sexual was those of us in the West do. For THEM, I wouldn't think seeing lady's bare breasts would be inappropriate. However, such cultures have lost something precious, as something so mysterious, sexual and exciting to men (female breasts) in the West, has lost its erotic context. And if public exposure of female breasts in public ever became legal and common, eventually men in the West would likewise have that same bored look as the tribesman surrounded by topless ladies - I would think those men would be exceptionally baffled by the Western interest in Playboy Magazine!