Any atheist who says rationality is the basis for whether or not we should eat or not eat an organism is not thinking this through unless they are willing to concede that it is okay to eat a human that is mentally retarded to the point where their cognitive functions are no better than an animal or a vegetable. Rationality has *something* to do with it but not everything. No non-sentient life form can feel any significant threshold of pain or suffering so it doesn't matter if they are killed for food or not and that appears to be our general rule of thumb in eating or not eating things (besides taste) but not the only rule.
We don't eat humans because a) our society has instilled the desire not to eat humans whereas some societies are cannibalistic b) our social contract teaches us to respect the dead so out of the duty towards our dead we do not eat them. There isn't anything wrong with eating humans in itself (unless you kill them but that's a different issue) but we have created a moral system based on our desire NOT to have our relatives eaten and that is reflected on our social contract. So yeh, humans > animals.
emotion + social contract + deductive reasoning = Ethics
Ethics aside, cannibalism is not an economical means of ensuring food supply or our survival as a species. I am not aware of any evolutionary advantage for humans to kill and eating their own kind. We are omnivores by nature and can survive on many different types of food, and have changed our dietary habits over time and environments.
Am I trying to cut back on meat? Yes. Do I see myself as an aspiring vegetarian? No.
All the best,
Faysal