I find this an interesting one. I would not have done what this guy did, but I also might not have handled it as the OP did.
Cultural norms are unimportant, in my view. They're subjective, they change all the time, they often don't represent the views of the majority, and they can be oppressive. It's the cultural norm in my industry, and in Britain in general, to drive to work, or take public transport. I prefer to use the train and my folding bike. I'm not the only one, but the VAST majority of people, including almost all my clients, don't do it. I get asked questions about it, called crazy or brave for doing it, and maybe sometimes it has an impact on my own career in a minor way, I don't know. I don't do it to be different or to be noticed, or to prove how I'm more green and liberal than everyone. I do it because I enjoy cycling, I need the exercise, it's better for the planet, a prefer snoozing and reading on the train to concentrating on the motorway, but I still like my vehicular independence when I get to the other end. I don't care much what people think about it.
I also like being naked, I find it comfortable, and I like the feeling of being liberated from the idea that the world will end if anyone sees my body. I know that nudity is not illegal in this country, contrary to the often stated belief here, so I have to make my own judgements. At a nude beach, or in swimming pool changing rooms in Germany, or in a sauna at a music festival, I'll absolutely be naked. In Tesco, absolutely not. There are a few places less simple though; I'm naked in my garden, but I checked that my neighbours don't mind. I might be naked at a music festival elsewhere than the sauna, maybe in my way to or from it, maybe sunbathing by my tent. At noon nude beaches, I don't cover up while changing (and I note that almost everyone takes this in their stride), and whether I wear shorts depends how many people are there, and who they are. If it's packed and busy lunchtime, I'll wear them. If it's quiet and people are spread out I don't, and in the evening of the families are mostly gone, I never do, and the dog walkers, strollers and barbecuers are, in my experience, at least as likely to join in as they are to tut.
I've done World Naked Bike Ride too, and there, hundreds of people clap and cheer and say "that's fantastic, I wish I had the courage, keep it up", and a tiny few get outraged. You might see 3 or 4 outraged reactions. All of this leads me to think that actually, lots of people are much more laid back about nudity than we assume. We make the assumption that everyone's life is like ours, so people who grew up with open nudity in their family assume most are like that, and don't feel like they need to tell people that think it's OK, whereas those who were never allowed to let the opposite sex see their pants assume that's the norm.
I would prefer to get out of the pool, return to my locker for my towel, leave my shorts in the locker and go take a shower. In a men only changing room I do this, in a mixed one I very likely wouldn't, and in a busy one full of kids, I definitely wouldn't, because of the likely reaction. However, if I came into a changing room and someone else was doing that, unaccosted by other people, I probably would even if others weren't, because it would be apparent that it wasn't a big deal.
In the OPs situation, context and behaviour are everything. My 6yo DS wouldn't be horrified, but she might be surprised, because she's learned that nudity is fine and bodies are fine, but some people don't agree. If the guy was just having a shower, with no sign of any other behaviour, I would just ignore it. If I felt he was at risk of being hassled, I would politely point that out. If I felt he was being in any way aggressive, I would tell him directly that he needed to stop, and if I felt I couldn't do that. I'd probably do as the OP did. Pretty much everything would rest on the song and dance she felt he made of washing his trunks.