Wally, our nine month old, 120 pound Newfoundland dog, likes to hump legs, chairs, cars… anything that will stand still for him. The breeder we got him from doesn’t want us to neuter him until he’s 18 months old due to research indicating that their growth can be disrupted due to hormone imbalance. Or something.
Unfortunately, he’s at the adolescent age where he wants to mate with everything, and when that something is a leg from a 130 pound human, the human can become flustered about dealing with a 120 pound bear-like dog. Fortunately, Wally is a gentle giant and a forceful shove gets him off, but still, it can be intimidating for people who aren’t used to dogs that big.
I think the main problem is that of dominance – he’s humping anything that won’t resist him.