The answer is simple: The Golden Rule.
If you don’t want it done to you, then you should not do it to others (beings).
I think you forgot to think of the law of kamma when asking this question.
“fixing” animals is popular in the United States, but not so in Asia.
I often tell my fellow monks in Myanmar that dogs in the USA are more celibate than humans.
Nobody believes me until I explain that many dog owners have one dog. If they have two, they are usually brothers, or the same sex, always on a leash and kept indoors, or ehem… “fixed”. The mouth usually hangs open in amazement for several seconds with that last part.
If you think breeding dogs are “lucky”, think again.
Dogs that are commercially bred (I know a breeder), are actually injected with sperm by syringe.
You don’t see dogs stuck together in the USA, but if they try to separate, it can harm the prized breeding dog. I’ve never seen dogs stuck together until I went outside the USA.
The USA does not have street dogs because The USA and even PETA kills them. If PETA does not end up killing them, they certainly “fix” them.