I’m not against dog parks, and I’d never tell someone to leave a dog park (if they were bothering me while I was at one), but sometimes as GSD owners we aren’t as vigilant as we need to be. Even though our dogs are playing normally, some people might not be comfortable with it, and unfortunately it really does only take a split second for a GSD to hurt a small dog. With a larger dog…there might be a puncture and an injury that hopefully can be fixed up by a vet, but with a small dog, one bite could mean death.
I’m not saying this to OP, but people need to realize that dogs mature and can very quickly go from friendly to angry. I saw this happen with my dog when he was right around a year old, and too often on this website I see people who are oblivious to the fact that their young dog will change at some point. Luckily, I saw signs of the change and stopped going to the dog park because it stopped being fun for me. Others though, don’t stop until it’s too late and then wonder why people are prejudiced against GSDs.
When I went to the dog park, I was very aware of the preconceived ideas about my dog due to his breed and would watch him like a hawk. I’d watch the owners of the other dogs he’d play with to make sure they were comfortable with the play, and you’d be surprised how many times I made my boy stop playing with a dog and had the owner tell me “it was fine” when it really wasn’t. Their dog was clearly uncomfortable with the situation and not enjoying themselves. Even though all our parks have small dog areas, people would bring their small dogs into the large dog area (it was way bigger) and I made sure my dog never played with too small of a dog. Basically anything smaller than 30lbs was off-limits.
I don’t think you did anything wrong OP, but just be mindful of the fact that your dog has a much shorter leash than other breeds of dogs. In this situation, you ran into someone that over reacted to the situation, but unfortunately there are a lot of those people out there. And if you ever read posts that come from people whose dogs were overwhelmed in that type of situation, the advice will usually be to “protect your dog at all costs.” Which is what that guy did. Is it a bit too far, especially in a public park? Sure…but that’s what happens when you’re at a public park.