Its an arbitrary line, holding slaves at least follows from the golden principle. What principle says don't raise animals in poor conditions or eat them? Are you extending the golden principle to apply to all animals? Why? Do you think these principles come from anything but a higher-order self-serving? Rather naive to make up a principle for no other reason than a poorly identified (non)-symmetry. I could see applying the golden principle to a dolphin but its probably some back of the mind idea that maybe one day my grandchilds grandchilds grandchild will be half dolphin and I should hedge for that eventuality.