I'd like to know where the phrase comes from. Was there ever an actual contest where two or more people competed in urination skills? And if so what was the goal? Length of the stream? Amount of fluid expelled? Accuracy in hitting a target?....
Hiya Nathan,
It just so happens after you brought this up, I did a quick search and came up with this little tidbit of information....
Its a Swap Thing
When two crayfish of about equal size meet, they rear up on their tails, spread their claws wide, whip each other with their long antennae, and engage in what is literally a pissing contest. Openings in their faces called nephropores eject streams of urine to communicate each individual's status, health, and identity.
Then the fighting commences. Gripping each other with crushing pincers, both crustaceans churn the water with their tails in a show of strength. Limbs may be lost in the heat of combat; like geckos dropping their tails, crayfish autotomize a damaged or entrapped limb by closing a sphincter of muscle at its base, and then regenerate it.

heres the link
http://nationalzoo.si.edu/Publications/ZooGoer/2004/5/crayfish.cfmCan't say for sure thats the actual origin of 'pissing contest' but its as good a guess as anything.
Speaking on topic, why is it we all can't look at audio like we look at women? Myself, I like brunettes, you might like blondes, somebody else might like redheads. Some like girls with long legs, some like blue eyes, some like girls with big butts and so on.
Why is it when we all talk about girls, when somebody says what turns them on and it happens to be different than own personal tastes, we accept it without question or judgement? It just happens to be their tastes.
I mean heck, can't we scientifically measure girls too? I'm sure I could find a way if my wife (and the girl under test) would let me.
Why can't we all look at audio like we look at women?
OK all you sick puppies out there, no going for the golden showers jokes...