I, too, have this problem though not nearly as bad as when I was in high school. Then, the restroom would have to be completely empty for me to piss. Now, I can piss in a stall no matter who is in the bathroom and can use a urinal if no one is next to me as I start the stream. I am former military and remember having to take a drug/pee test one morning where you are REQUIRED to pull your dick out and pee in complete view of a witness. I remember being anxious that morning but got through it by drinking three cans of soft drink and needing to piss like a race horse. Thankfully, I didn't have a boner while doing it either, though, ordinarily, I would have had.
All said, this problem is a pain in the ass. Even now, I still would not be able to walk up to a trough at a football game, whip it out and whiz right next to someone.