Why do men wash their hands after urinating? It's not from touching the plumbing—many of us have partners who'll stick our unwashed organs into their mouths with only positive results. And anyway, urine is sterile. So what's the point? —A Concerned Citizen
There is actually a legitimate reason for this—but first, Concerned, allow me to congratulate you for taking what clearly began as a boast about how much fellatio you've been getting lately and managing to dress it up as a legitimate scientific inquiry.
You are correct that urine is sterile. Indeed, if we humans greeted our friends by splashing them with a brief stream of urine rather than by shaking hands, hand-washing would be unnecessary. The problem is the other things that go on in your underwear.
Now comes the part where I ruin your life forever by introducing you to the concept of perianal sweat. That's when perspiration from the area immediately surrounding your nether orifice drips down into your briefs, rapidly turning your entire package into a sweltering biohazard.
Epidemiologists insist that this happens to everyone, even if you won your high school's "driest anus" award four years running. Thus, it's only polite to wash your hands, just in case.
Interestingly, every source I consulted on this subject felt the need for a sidebar on the topic of oral sex. All had the same argument as to why it was OK: The human mouth (the story goes) is actually well-equipped to fight off the kind of bacteria we're talking about, at least when they come in the relatively small amounts you'd find on somebody's junk.
So if your mouth is so great at repelling infections, why do we worry about bacteria at all? Supposedly, it's because the real danger comes when the bacteria are transmitted from your package to your hand to a food item, where they can multiply into dangerous numbers.
And there you have it. Still, if you think that last part sounds like a desperate carve-out from a male microbiologist who just realized his wife might get around to reading his paper, I'm not going to argue with you.