Overpopulation that Life mentioned is good reason, but perhaps not for someone who comes up with this belief in the first place! Arguing "Human Rights" for human's over-population IN ANY WAY, deserves argument. Many animals, not just pets, are over- populating or trying to hold onto space humans have take over. It creates problems for their own species or humans, so some have hunt "seasons" on killing them, or there may be size requirements. It's control for the good of the people and the animal who may wind up w/o enough food/etc. In my opinion spaying should be considered the same, but I am taking the belief at face value--in which case it may be unnatural. However, there is nothing inhumane about surgery of any kind on any pet if done by a pro. Animals are not left with needs or sensations which could be called inhumane in people, who have other sensations. Being a "pet" is unnatural to most animals, even cats and dogs. Responsible breeders often wind up w/even show dogs as real pets, and they accept that responsibility. Irresponsible breeders should be shut down. Most people just have a pet. So what? It's the owner's responsibility to keep the public and other animals safe and/or "happy;" being a pet is already "unnatural" to some species. Dogs and cats, definitely yes--expenses and/or the unknown future, possibility of passing on dangerous diseases not automatically vaccinated for, over- population, etc, is the owner's responsibility.