Having volunteered at an animal shelter, it's not so easy for me to dismiss these types of comments as jokes. I know first-hand that many people see pets as disposable. I have seen people surrender cats -- including elderly or sick cats who are unlikely to get adopted (and it wasn't a no-kill shelter) -- for such reasons as "my new girlfriend/boyfriend doesn't like cats" and "I got new furniture and the cat's fur doesn't match." Really. Eliminating outside the litter box is probably the most common reason people give for surrendering their cats, and they know full well that the cat will most likely be put to sleep since a cat that doesn't use the litter box is not considered adoptable. So I would not be inclined to think people who say those things are joking.
I would also not want to respond with something like, "different things work for different people." As an animal lover, it's similar in my mind to someone making racist or sexist comments, in that it should not be tolerated or ignored in the name of politeness. If my DH complained to a friend that I never did the dishes, for example, and that friend responded that he would give his wife a black eye for not doing the dishes, I would be horrified if DH's response was to shrug and say, "whatever works for you." I realize that not everyone sees animals the same way, but for those of us who do, I actually think it's wrong to let such comments pass unchallenged. It's one thing to give a mild answer, such as "I disagree," rather than engaging, but tacit acceptance is different IMO.