Unless we are talking sub zero Arctic weather a man's need to take a leak always outweighs the temperature of his hand. Granted it's more fun to take a piss in a climate controlled room than say, in Rochester, NY off the side of the highway in February, but we are a hearty people, us men.
Regarding aim, don't give the hand that much credit. I mean, yes, our hands do ensure we don't piss on our shoes, or knees, but the wang design itself, armed with the natural trajectory of steady urine flow, pretty much does the trick.
What's more, with the advent of gloves and adult diapers we are fail proof. Our bladders don't stand a chance. Bring it on, Ice Age!
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Actually, I'm quite capable of going pee while touching nothing but my zipper and the elastic band of my underwear.
I think the only way cold could interfere with peeing is if the hands are frostbitten enough for one to lose motor functions so he's not able to work the zipper. The small discomfort of touching the penis with cold hands is not even something we notice except extreme cases.