[Britster]

Humping is not always dominance. In fact, most of the time it's not. Rather, it’s often a social behavior OR even a stress reliever. Humping other dogs can be about dominance, control, etc. Usually younger pups, it’s about play and social learning.

A dog can easily get overstimulated during play. Overexcitement, etc is often a cause of humping. Jackson has never humped another dog, a human, a leg, lol, anything. When he's super wound up and excited, he will come running into my room and start humping his teddy bear for fun, LOL.

I am being completely serious too when I say that dogs... masturbate. True story! lol.
There’s no real harm in it, as long as the objects used are reasonably appropriate (say, a washable stuffed animal that belongs to the dog instead of... your couch cushions or something). Of course make sure it doesn't become an obsession either. If it's causing harm to your dog in any way or they're so overly concerned about it, you will need simply remove the objects and do some behavior modification. But otherwise, I don't see the big deal.