I think voyeurism is implied to mean "looking at something you shouldn't be looking at" so even if the person is an exhibitionist jacking off, you are looking through a peephole, right? Therefore you are voyeur. I suppose if he was leaning up against a sink jacking off and you walked in the bathroom and watched the whole performance, well you'd still be a voyeur since you ostensibly went to the bathroom for another purpose, right?