Why are dogs that are not neutered at a dog park? Dog parks in my area do not allow any dogs that are not fixed. I would check the rules for this parks and see if the dogs are allowed and then maybe point this out to the owners.
Really, the only answer I can give is to work on socialization or go during times when these dogs aren't there.

 

I think it is very uncommon to not allow intact dogs into a dog park. That would essentially mean any dog that is a show dog, used for breeding, or owners who simply make the choice to not alter wouldn't be allowed in.
I think WAY more common is that females in heat are prohibited, that sign is pretty much on every dog park. Anyone who brings a female in heat to the dog park is a nutcase.

To the OP: Is your dog neutered? If he is, it could just be he doesn't enjoy rude, pushy, dogs. I try to take my guys over to an area of the park away from everyone else if there is a pushy/not nice dog. If they follow us, I go over to the smaller separate area (usually reserved for small dogs, but there are never any around). Dog parks are so hit and miss, it is difficult to say what kind of experience you'll have when you go.

 

I don't take my very large intact dog to dog parks because he was constantly being harassed by neutered males. His puppy privileges expired when he hit around 10 months old or so, after that he became a hump magnet.