Sumi always mounts Casper but she is usually never dominant toward any of other dogs she meets. I guess she wants to make sure he always remembers that she's top dog and was here first. I think it's sort of funny being that she's the girl ...plus she's fixed ... plus often she get's his head where his butt should be. Poor Casper is usually just standing there as if to say "I'm just trying to play fetch here guys, could you stop lauging at me long enough to end this rape?"
I know that it bothers some people though, I remember we had a client over once and he looked over at her just as she decided to, well, do it. And he got this stricken look on his face and started saying "Hey stop that!" and "Cut it out." Even though they weren't even his dogs. I guess he was sort of prudish or something. I don't see what the big deal is, I mean they are dogs not people. I think that's sort of one of the nice things about dogs, they can get away with farting and burping and humping, shamelessly.
By the same token if I had a dog that didn't go around doing this on his or her own, I wouldn't encourage it ... that seems an odd notion to me. If your dog is not doing this naturally, trust me, it is nothing to wish for.
I have no idea if there is a way to stop your dog from doing this at the park. I notice that when Casper gets sick of it, he just leaves or tries to mount her in return, so it seems to just be part of dog communication to me.
------------------
mama to:
-Sumi, lab mix "pound puppy"
-Casper, white boxer rescued from the Dade County Animal shelter