P.E. at my high school wasn't really taken seriously. Sure, it was a requirement every year, and it was held twice weekly, but it was never an intense workout. I remember playing basketball or volleyball in the school gym, but it was rare that anyone would really break a sweat. After class, we were usually given just enough time to retreat to the locker rooms and change back into our school uniforms -- not enough time for even a quick shower, and while there were shower facilities there, no one ever took advantage of them. So, in that respect, I didn't get the true "gym shower" experience.
However, I swam frequently at the local YMCA, starting at a very young age, and I'd always shower after getting out of the pool there. When I was younger, I'd keep my bathing suit on in the shower, but I was always somewhat in awe of the "big kids" and men who would shower nude, unembarrassed by the exposure. I eventually evolved into one of those "big kids", and showering in that setting didn't really phase me, though I'll admit I tightly wrapped my towel around myself when I got out -- which was not a practice shared by all Y members, especially some of the older men who seemed to relish lounging about in the locker room while undressed.
My most memorable group shower experience was at summer camp with the Boy Scouts. I don't recall how old I was at the time, but it was likely middle-school age or at most freshman year of high school (I stopped participating in scouts activities when I turned 16). The summer camp I attended with my troop was a week long. We slept in tents, but the campsite included a simple shower building with a large changing room with benches adjacent to a similarly sized shower room with about a dozen showerheads. My troop disliked the troop at the neighboring campsite who shared the shower building with us for no other reason than proximity and that's what immature kids do. They took showers in their bathing suits, but we saw ourselves as more mature and grown-up for such things, so we showered nude, which made them uncomfortable, and they'd wait until we were finished to use the facility. Each night, we attempted to use all of the hot water before they could get a drop. I remember being a bit apprehensive about going nude. I wasn't attracted to any of my other campers, but I was a young boy prone to getting erections at inopportune times. I managed to stay calm at each shower, but I recall one of the other campers frequently getting erections in the shower. Interestingly, none of us gave him a hard time about it or even acknowledged it (probably knowing that it could have easily been any of us in that situation). Looking back, it's funny how we could be so immature as to hassle our camp neighbors and so mature at the same time as to not tease our friend.
Of course, I never really associated showering with medical things, even if they shared the common thread of nudity. I do recall a thought I had as a young child, though. I was probably in early elementary school -- 1st or 2nd grade at the time -- and changing in the locker room after swimming at the Y. I pulled on my underwear while turning toward the lockers, trying to keep covered under my towel as much as I could. Suddenly it hit me that those were the same underwear that I had wore to the doctor a week or so prior. I was exposed the same amount, but at the doctor's office it was so straightforward -- so acceptable to be on such display, while in this other context, while I was dressed exactly the same way, I had to be modest and do my best to keep from being exposed. It was around that point when I realized there was something special about going to the doctor -- it was different from all other interactions with another person and all other instances of being undressed.