[Twisol]

I had this thought too. My best guess is that it's down to the length of the urethra. Have you ever lifted liquid out of a glass using a straw, by capping the upper end of the straw with a finger? That liquid wants to fall, but that causes the gas volume above the liquid to expand, so vacuum pressure pulls the water upward, counteracting gravity, and also pulls your finger into the straw slightly. I imagine that, for a longer column of fluid, that vacuum pressure is at least a contributing factor to the unpleasant feeling. (Not to mention that, when urinating, that column of liquid is already moving rapidly!)