Greek 4 said it best - humping is not sexual or some perverted game - it is all about dominance and establishing hierarchy in a pack. I never realized there were so many who were viewing this as human behavior - not thinking like a dog
It has nothing to do with whether a dog is fixed or not - fixed dogs, both male and female, mount others for dominance all the time. When bringing a new dog home, it will be especially common as they sort out where each other ranks in the home. I would never interfere with this in the home - it is critical that they sort out who is leader in the pack and the worst situation that can arise is ambiguity about it - that causes friction and in some cases even fights. When stopping it, you are saying "no, its not you" - when maybe it should be and would be. They need to sort it out.
Outside the home is another story - I used to be of the mindset that my dog had to learn to defend himself and learn how to "get out of it". Now I simply don't let another dog mount mine, as it is a "bully" tactic - the big dogs will always get challenged and mounted by others because they are so big - it is the mindset of "well, he's huge, so if I can dominate him, then I am the king". Don't allow that stuff.
My golden is fierce about getting dominated - when dogs walk up to her to mount her, she'll scream at them hard if they dare. My other dog, a Berner however, plays everyday with her best friend, a girl, who loves to hump her occasionally during the play to announce "I won - I'm dominant"
In the ends, its a fundamental pack behavior.