So you've neutered your male cat, which should end any problems with spraying, aggression, and other such behavior, including sexual behavior. Perhaps you had him neutered young enough so that this never became a problem to begin with. But neutered male cats don't lose 100 percent of their sexual instincts. They're just less pronounced, oftentimes enough to appear to be gone, but not always.
When it comes to mounting your female cats, if he was able to mount a female cat before he was neutered, this behavior might continue even though he's neutered, according to Veterinary Practice News. This is especially true if he's exposed to a female in heat. The part of the brain that's activated because of testosterone isn't de-activated through neutering, but simply less active, due to the reduced levels of testosterone.
Author Nicholas Dodman, who wrote the piece in Veterinary Practice News, likes to use the analogy of a dimmer switch. Sexual behavior and aggression are dimmed, but not off. So you might see your male cat "play-mating" with some of your female cats. He might also be chasing them away, to assert his dominance in the household.
Amy Shojal, of Ask Amy over on About.com, says that humping behavior is normal, even in a neutered male. She says that this activity usually upsets owners more than it upsets the other cats. However, this could also be a sign of the start of reinforcing social standing in your house. If your male kitty stalks and mounts your females, and then chases them away from you, or the litter box, or food or water bowls, this is likely what's happening.
The problem here is that we often try to make things fair for our cats by not playing favorites, and hope that will make the submissive cat stop being submissive, and stop allowing the male to mount her. We also hope that by not playing favorites, the dominant cat realizes he's nothing special and stops terrorizing everyone else. This often isn't the case, because cats are cats and don't have our sense of fair play and equality.
Her advice sounds counterintuitive, but she recommends that you help to reinforce your male cat's social status. If your female cat runs away, she's giving up the resource your male cat demands, even if that's you. Give him treats first, feed him first, let him have your lap first. It might reinforce his social standing to the point where he no longer feels the need to beat the point to a pulp, either by beating up on her, or by mounting her.
If he doesn't stop mounting her, though, as long as he's not hurting her, there's really no harm to this behavior. If she tolerates it, there's no real reason to try and make it stop.