When you start to perspire, even a little, sweat from the perianal area starts dripping around in your underwear, eventually getting into the fabric and moving onto your genitals.
"The point is that simply touching the penis in an effort to direct your urine flow can be more than enough to transfer harmful microbes to your hands, and then on to the pretzels sitting in bowl on the bar," says [biology professor] Fidopiastis.
Fidopiastis adds that there may be instances when we needn't wash our hands after peeing. "If you can urinate in a hands-free urinal and pee without touching your penis, can you not wash?" he says. "I guess my answer there would be a half-hearted, 'Sure, why not?'"Does the argument for washing even apply to women? And yet I bet more women than men wash their hands.