When a dog gets fixed, he isn’t making the decision for himself. Animals act on instinct alone, but mankind has the use of intellect, reason, and will. Animals do not.
A person could deny themselves food in order to make a political statement or some other reason even to the point of death. An animal would never do such a thing.
Man can know that the primary purpose of sex is to procreate. Animals do not have the intellect to understand sex, they just have the impulse to act on the instinct and sex drive.
Because of the intellect of the human person, we can know that sex is primarily for procreation, and secondarily for unification of two persons with the possibility to form the core of a family if they have children. Therefore any sexual act in which it is anatomically impossible for procreation invalidates what we know by reason what sex is for. You could say that this is using the sexual organs in a manner which we know only fulfills the secondary purpose not the primary. It’s a misuse of the body. It would be kind of like having a meal with friends, chewing the food, but never actually swallowing the food. Thus the primary purpose of eating (nourishment of the body), is never fulfilled in the act of eating a meal with friends. It defeats the purpose.
Comparing humans to animals to justify behavior devalues all of humanity. Animals commit rape, but we know that that is wrong and we could never justify a person commiting rape because man is more than any animal. We have intellect, the ability to love, and the will to deny ourselves for a greater good.