If I find a guy physically attractive, I might try to visualize him naked. What he's wearing doesn't matter. Curiosity, maybe.
If I see a hint of a bulge, or can see nice definition through a well-fitted shirt, it's totally deviant and sexual. The visualization turns to fantasy.
If I'm talking to a guy I hadn't found attractive, but his intelligence and wit has me finding him attractive and sexy, I will definitely try to get the picture.