is a form of sentient bacillus culture, and that the idea of eating something so very like them is anathema. At which point Earth withdraws from negotiation, and passes a law decreeing that everyone has the right to eat whatever he damned well pleases.

I'm vaguely remembering a story that reverses the premise. Earth makes contact with a galactic civilization. Which reacts in horror when they find out that domestic turkey looks exactly like alien-race-1, which is a long established and well-regarded member of the galactic alliance. Earth agrees to no longer eat turkeys, when it is discovered that cows look exactly like alien-race-2, and the chickens, and pigs, and fish, etc. Eventually, humans decide that they'll need to be vegetarian, "but at least we can still eat yogurt", to which the galactics react in horror, explaining that alien-race-X