Which do you think takes a bigger toll on the environment, owning a dog, or owning an SUV? My bet would be on the dog. I'm thinking of all of the resources that go into dog food.
I was almost surely wrong.
However, if you believe that carbon is underpriced, so that carbon-intensive consumption (the so-called "carbon footprint") is a negative externality, then there is a large negative externality associated with dog ownership.
My guess is that people intuitively over-estimate the harm caused by SUV's, particularly when the alternative is a car that consumes at least 50 percent as much gas. My guess is that people intuitively ignore any green-hostile aspects of owning a dog.