A neutered male dog may become sexually aroused post neutering but cannot breed with a female dog. Neutering diminishes arousal in most dogs, but there is the possibility of a neutered dog becoming attracted to a female in heat and trying to mount her.
Neutering may stop the dog from mounting other dogs or people, but mounting isn't always a sexual behavior in dogs. Some dogs mount other dogs as a way to show dominance or as a form of canine play. Arousal in neutered dogs can result in an attempt to mate, but even if he is successful at mating, it will not result in a litter.