I can see it as a humilation thing, much the same as being peed on, but that wouldn't explain why it's so ubiquitous when watersports aren't. We'd have to be talking a minority of the population for that, certainly not enough to put it in so many porn flicks.

Maybe it's only well balanced, educated, chilled out guys who don't have any power issues with their SO's who don't go for facial come shots, which would explain why I don't know the people who do :-)