Mods please don't delete, if I google my question all I get is gay porn

I'm too shy (plus an "extreme" grower) to whip out my dick in any public space unless it's a closed toilet cubicle.
So, I never use a public urinal unless I'm one of the last people (or likely the last) in my office, i.e. I'm reasonably certain I will be alone.
Now, here I am, at night, alone in the bathroom, I whip out my dick, and try to use the urinal. And I sit there with my dick in my hand trying to pee but I can't. And I'm slowly getting a boner, until I end up peeing all around the urinal.

Why would someone get boners when using urinals? Am I the only one?