Well obviously you are bothered enough to actually do a thread, I'm sorry...I can definitely imagine how embarrassing and stressful it is if they do this in front of people not to mention that I wouldn't be too thrilled to see it myself on a regular basis....at this point, for lack of better info. I would separate them unless you are right there, start from scratch and monitor nonstop for a couple weeks always quick to STOP the behavior...I would consider the No Bark Spray or water spray bottle and firm NO's....here is an article from the SPCA it seems to include both fixed and unfixed dogs. Good Luck!
ASPCA | Mounting and Masturbation