Have you ever been to an animal shelter and seen how many animals are euthanized annually? Or should I be "ethical" and call it killed, gassed, or injected with lethal drugs?
Do you feel it is ethical to kill unwanted pets? If not, then what is ethical, and how do you propose we deal with the overpopulation of unwanted animals?
With an estimated 2.7 million animals KILLED, every year in the United States alone, what alternative to you see to this "ethical treatment" issue? That is nearly 8,000 animals per day. I truly don't see the "ethics" in this.
Besides this, animals make better pets if spayed or neutered. You have apparently not been in the same house with a cat who is in heat. Nor, have you had a male cat spray on your furniture. Or had a cat or dog run away only to be hit by a car, because it simply wanted to find a mate. And, then you tell your kids: "Fuzzy went to a farm to live", likely because you don't want to tell them Fuzzy was run over by a car, when he got out last night. Ethics runs a thin line, but cruelty and fairness have a much wider and unfortunate gap.
If you want the real answer to this, query any animal rescue league. They will fill you in on the "ethics" of your question. I will warn you, however, they are not "shy" in their opinions.