This is a strangely difficult one for me to answer. I have voyeuristic tendencies. I enjoy looking at young men but this enjoyment, to me, is largely aesthetic rather than prurient. In other words, it doesn't necessarily make my dick twitch. Rather, it is just a form of fascination, like looking at so many beautiful things I could name. I just want to watch them and enjoy them from affair. Of course I'm reluctant to do this except in certain situations (like watching the dive team practice at the pool) precisely because it almost invariably makes them feel uncomfortable.
What turns me on can be any number of things. I especially like watching guys who are together touching one another. Maybe they are holding hands or have their arms around one another or whatever. This I find titillating. If they kiss it starts to get me hard. But beyond that, beyond the voyeurism, what gets me excited is touch. Touching or being touched really does it. Watching a guy jerk off on cam works, too, but, honestly, its no fair subtitle for tactile sensation.