It is not mating behavior, but dominance behavior you are seeing. One chin mounting another is a way to express dominance. So Stuart is letting Spike know he is in charge. It is very possible that the presence of the females is causing this behavior to be more pronounced in your boys. I would watch them carefully to make sure they aren't fighting or showing signs of aggression towards each other. Competing males have been known to fight viciously, even to the death.
