!! OMG how painful: Priapism !!
Okay don't ask me how, but, I came across THIS WIKIPEDIA ARTICLE about "Priapism":
Priapism (Ancient Greek: πριαπισμός) is a potentially harmful medical condition in which the erect penis does not return to its flaccid state (despite the absence of both physical and psychological stimulation) within about four hours. It is often painful. Priapism is considered a medical emergency, which should receive proper treatment by a qualified medical practitioner.
The name comes from the Greek god Priapus, referring to the myth that he was ironically punished by the other gods for attempting to rape a goddess, by being given a huge (but useless) set of wooden genitals.
Yowsa! I thought having a constant erection might have been fun...sometimes inconvenient...but fun. I guess I was wrong. I'll never think of the PRIAPE porn store the same way now.
See the full possibly NSFW painted depiction of Priapism after the jump. Looks like this is where Tom of Finland got some of his ideas.