We don't need meat specifically, we need animal protein. Meat is not the only source of animal protein. Besides, I and all humans do a billion unnatural things daily, starting when I get out of my bed and brush my teeth, dress in clothes, drive my car to work, etc. If we were bound to doing only natural things we wouldn't be having this conversation. As a side note, dogs aren't universally recognized as true carnivores. They can digest and utilize some plant material.