Now, on a serious note -- why do you ask? Is there someone you want to piss on or are you hoping to be the pissee?
It happened this weekend at Dude Fest between to this guy I was giving a ride to from one of his friend's friends, a girl. And has basically become the top story now out of it. It was done sort of as a way to codify his victory in the unofficial contest over something by getting a tattoo referencing an inside joke on the message board.
Ah, I see.
Well, to seriously answer your question...I would be primarily concerned about health and safety. And there are actually people who take it a step farther...particularly those into the whole "humiliation" scene. Not only do I fail to understand the kink (and I like some kink) but I would be concerned about diseases and infections.
It's noteworthy that you cite a male-female situation. Few things enrage my gay friends as the ignorant suggestion that such practices are endemic to the homosexual lifestyle. Millions of straights do things like this, too. And those who say otherwise are about as clueless as old Immadinnerjacket.