Fallacy
Humans evolved as an omnivore species, so eating meat is natural for us.
Response
The claim that humans are natural meat-eaters is generally made on the belief that we have evolved the ability to digest meat, eggs and milk. This is true as far as it goes; as omnivores, we're physiologically capable of thriving with or without animal flesh and secretions. However, this also means that we can thrive on a whole food plant-based diet, which is what humans have also been doing throughout our history and prehistory.
Even if we accept at face value the premise that man is a natural meat-eater, this reasoning depends on the claim that if a thing is natural then it is automatically valid, justified, inevitable, good, or ideal. Eating animals is none of these things. Further, it should be noted that many humans are lactose intolerant, and many doctors recommend a plant-based diet for optimal health. When you add to this that taking a sentient life is by definition an ethical issue - especially when there is no actual reason to do so - then the argument that eating meat is natural falls apart on both physiological and ethical grounds.
Issue Responses
Humor