I've been in a great many fandoms and I must say that for money shots I've really only seen it come up frequently in Supernatural. Though, as fandoms go, Supernatural is by far and away the kinkiest of those I've been in. I've always assumed it had something to do with the fact that it has the greatest majority of slash and incest readers/writers. Maybe it's breaking that first taboo or maybe it's the subject of the canon, the flexibility to be able to make whatever cracked out kink you have come true that does it.