Actually, I would think it is all behavioural.
Mounting in neutered animals is really quite common. Neutering usually eliminates the behaviour, but not always. A lot of the time mounting is a dominance behaviour rather than sexual, other times it is because animals are just hard wired that way.
Mig the bun has tried humping me a couple of times, years after his neuter. Male dogs can and sometimes do get their wicked little way with the ladies even though neutered. They go through all the motions (sorry
) even getting a tie just they can't sire a litter.