What would you show a curious man to get him horny?
I'd start with a nice, smooth butt. Everyone likes asses, especially nice ones. There doesn't even have to be a dick in there, keep them guessing.
Then I'd follow up with something more explicitly male. I'd choose something without a face. There are lots of people, especially bi-curious men, who actually like cocks, but don't necessarily want the man attached to it.
If they're responding well to that, I'd continue with pictures of cum, maybe some pictures of twinks fucking. They seem to be a nice way to get started in the world of gay porn.