Seriously though, I'm sure you don't need to have an erotic dream for it to happen. A sex ed instructor way back when told the class it happens when a guy has a sexylicious dream. But then a bio teacher said it would happen regardless of the nature of the dream you had (or if you dreamed at all), that when your body is ready, it happens. Funny I never payed attention, waking up satisfied with a nice warm goo spot is just mundane, no need to recall the dream (if any).