What is the difference between looking and lusting?
Lust is a disordered desire for sexual pleasure. It’s necessarily self-centered and objectifies the person desired (CCC 2351). This is why Jesus says a man who looks upon a woman lustfully commits adultery with her in his heart (Matt. 5:27-28). Or fornication, depending on the circumstances.
Looking doesn’t necessarily imply lusting but could instead mean appreciating someone’s beauty as a whole person. However, men especially should be wary, because the longer one looks at a woman not his wife, the more vulnerable he becomes to lusting, and the progression can be rather quick.
In addition, to circumvent trouble, I suggest men instead may notice the beauty of a woman, but then quickly move on. The more a man grows in the virtue of purity, the easier this discipline will become (CCC 2520-27). Yet, on this side of the grave, any mere man will never be completely free of temptation in this moral area.