I'm not big on voyeurism myself. The idea of "just watching" is frustrating to me. I have had my fill of looking at appealing men and being turned down by them because I'm imperfect or too intimidating (e.g., I have a brain).
But others get off on it. And I find that dynamic of "performer and watcher" erotic. Most especially, I like the dynamic of the unexpected but not brazen: The idea of a naked or mostly naked man being seen through a window. Being nude in Times Square is brazen; there's a defensiveness that goes up in a man when he's naked in such a public place. But the idea of being in one's own apartment, being seen, all defenses down... that's erotic.