dominant behavior or problem?

Question
I have a 3-year-old Golden Retriever that is usually very friendly and laid back.  He is neutered and about 75 pounds.  We go to the local dog park often to play and he does fine with other dogs except when a dog tries to mount him.  When this happens, he growls, snarls, and barks at the dog. The dog usually backs away and only once has this resulted in the other dog fighting back. This never happens in any other instance such as battling for toys or food. He did have TPO surgery for hip dysplasia when he was 1 and although he runs and plays like a normal dog now, I worry that he may be slightly sensitive about his hips b/c of his past.  It is somewhat embarrassing to hear him snarl at the other dogs when they try to mount him but more importantly, I am not sure how to act.  Is this a normal battle for dominance situation, in which I should let it play out or should this behavior be reprimanded?  I almost want to avoid the dog park b/c it is bound to happen at least once.  Thank you for any input.

Answer
Most dogs see attempts at mounting as an aggressive, dominate move.  Most will react negatively.  Watch how the other dogs at the dog park react.  I see the dogs that try to mount other dogs as the problem, not yours reaction.  

Mounting is definitely about status.  When we get together with others, the worst dog about it, is our 11 year old, spayed, retired dog guide.