I started being pee shy in seventh grade when we went to the boy's room one day and I saw my classmate, Michael Jordan (no, not that one) taking a leak. His penis looked a lot bigger and hairier than mine and I felt embarrassed that I wasn't nearly his size. He was also circumcised tighter than I was and didn't have any skin covering his glans as I did nor a prominent scar as I also have. I felt my penis didn't look as mature or as normal as his did. From that point on, I began avoiding using the urinals when we went to the bathroom as a class and it got worse from there.
Oddly enough, two years later, I went to boarding school where the dorms and gyms all had gang showers. I got used to those, despite initial dread, very quickly, but still couldn't pee in front of anyone else! Even if I was in the shower and walked over to the urinals buck naked, I simply could not pee.
Pooping was definitely a toilet training thing. Even when young I couldn't use a public toilet to poop. My father never did and somehow I got the idea that one should only poop in a private bathroom.
It really wasn't a problem though until I saw Jaws and then became terrified of actually sitting on any toilet. Jaws convinced me that there might be a shark in the toilet and so I had to poop by hovering my butt over the toilet, so any sharks or snakes or rats wouldn't bite me in the ass, for a good 4-5 years before that fear finally subsided. I couldn't imagine doing something so ridiculous in public and so never used public toilets to poop. Once again, boarding school offered me no choice and so I eventually got used to going while sitting down, but it wasn't easy.