He acknowledges that animals can suffer: violation of humanity when we use animals in a way that are not necessary for our needs; it"s a violation because as humans, we have the capacity of empathy for suffering. So if you have no empathy to animals, you likely don"t have empathy for human suffering. Men and women are not significantly different in their interests. Without a difference in interests, no difference in consideration is justified. Anything that can suffer has an interest in not suffering. Humans and non-humans share an interest in not suffering. Without difference in interests, no difference in consideration. Causing animals to suffer in the way we would not cause humans to suffer, is unjustified. If you buy the equality argument for women, then you are logically impelled to buy the argument for equality of animals: equality as a moral idea, not an assertion of fact.