Why does my dog want to hump my leg?

My dogs and I play together often. I am the dominant of us three when we play. I’ve grown up with dogs my entire life, so I am familiar with dogs and their behavior. Every so often my make Great Pyrenees likes to try and hump my leg or face or side or whatever body part he can get a hold of when we wrestle. I grab him and sit on him and tell him “no” when this happens. Why does he desire to do this? Is it because he wants to show his dominance in the pack or does he really just want to make human puppies with me (which I think would make for a great horror flick)?
No he is not fixed & no I am not fixing him.