I've noticed when we go to the dog park, there are certain dogs that try to hump most of the other dogs, including our much larger male lab mix. Why do those dogs do that?
The dogs just "show them who's the boss".
Unfortunately many interpret this as being gay or aggressively offensive, which simply isn't true. Of course there could be more behind the whole thing in a male/female combination, but usually it's really just to mark dominance and part of playing/socializing.
Same is true for dogs trying to hump human legs. If one does, just shove it away (just like other dogs do), unless you want to drop in rank.
So if other dogs try to hump your lab, just let it solve this "issue" on its own, unless the dog can't or they're not properly socialized (i.e. they don't know when to stop or are somehow hindered).