Spaying and neutering, often called "altering" as a means if sugar coating it, are genocidal traditions of mutilation. The practice is nothing short of barbaric and inhumane, based on fears which were outright lies to fuel an artificial demand for nonhuman companions. The result is the emergence of puppy and kitty mills.
First I will address the baseless and hysterical fears used to justify this inhumane practice. The most common one is the fear that the nonhuman species will over populate. While humans are a good example of a species which actually has done this, humans are neither willing, or allowed, to be neutered to control our population.
Doctors and politicians are actually encouraging us to over populate more, meanwhile we rape the environment and displace other animals then turn around and slaughter the other animals when they try to return to their natural homes. So it is highly hypocritical for any human to ever say we should control the populations of any other species for any reason.
That's the biggest problem, but only one of many for this issue. Another baseless fear is that other animals will bring diseases to us. News flash, we have vaccines now and most diseases are not transmissible across species. In the past, other animals were regularly blamed for transmitting diseases that they are incapable of carrying. The real fault was our own, poor hygiene and lack of medical science were the biggest offenders.
Today, the few trans-species ailments are easily controlled by medications and proper precautions, so it's still our own fault if we do get hurt by them. Our species is one of the filthiest species on the planet, we breed diseases, viruses and bacteria, by our own existence. Yes, we do this to ourselves more often now because of communal living.
But what about the environment? Seriously, I have heard people suggest that allowing those animals who want to be wild alone can cause problems for the environment more serious than our cutting down trees and draining swamps does. Nope, no species could ever cause the harm humanity has, even the dinosaurs did not come close to the damage we have caused.
We have been selecting these species, cats and dogs, based on cosmetic traits for centuries. The result is a lower fitness. They need to be allowed into the wild so nature can correct the problems we created for them. This is a tough love issue many of us nonhuman care givers will cringe at, but it must be done.
Their wild populations will suffer a lot, many will die of preventable illnesses, but the mistakes we have made in their breeding will be corrected by nature eventually and they will find a place in the ruined world we have created. We should be sharing this land with all other animals, not destroying them just because we're too weak to control our own population for the sake of the entire planet.
I touched on the puppy and kitty mill problem, I need to get more in depth with it before closing. Spaying an neutering these species fuels the demand for the unaltered ones, which produces a huge demand for the mills. A mutilated animal has a high risk of several health problems, obesity being one, depression being another very common one, the imbalance of hormones makes them more prone to other issues like acne, infections, infestations, and personality disorders.
The shelters then have to require a huge payment to adopt them, large enough to make puppy and kitty mills extremely profitable. Profit makes human society do the most idiotic things, including torturing other animals. Many vets get paid by the mill owners to perpetuate this cycle by enforcing the mutilation, like seed companies demanding the unused seeds be destroyed, it creates a perpetual market.
So this is the moment of truth, for any human who has their genitals in tact, it is a vile hypocrisy for you to even consider doing this to any other animal. You are no better than the likes of Mao and Hitler.