I'm sorry you had a bad experience at the dog park. Those people do sound obnoxious. I hate encounters like that, which is why I prefer not to take my dog to the dog park. My GSDs are great on leash around other leashed or under control dogs, but I'm fairly certain that they would push back if they were confronted by a pushy or aggressive dog. And you know which dog is going to take the blame if anything happened--the GSD, of course.
I train my dogs outside the dog-park, and about every other time I'm there I see scuffles inside the fence (never anything serious yet, thankfully). Often a GSD is involved, and I just shake my head. Some breeds are more likely than others to enjoy that kind of setting. I think that GSDs being on the loyal and protective side, have no desire to form numerous indiscriminate bonds w/dogs that they have never met and may never see again. Of course, lots of people go to dog parks. I see a volatile mix of breeds in our park every day, including GSDs, and sometimes there are no problems. I like meeting other dogs, and I admit it looks like fun, and I'm tempted to go in. However, it's a risk I prefer not to take as I can't control all the variables. Fortunately, I have a fenced yard for our dogs to run in. If I had to go to a dog park, I'd probably try to go very early in the morning before other people got there.
Anyway, I'm annoyed also when people say stupid things about my dogs--I can think of a couple examples, but I'll spare you, since my post is already too long.
I think when people make stupid comments, it's a reflection of their own insecurities and previous interactions with GSDs, not anything that you or your dogs have done. I just try to ensure that my GSDs are as polite and well-mannered as possible wherever we go so that we can be good breed ambassadors.