Vegan humanoids visit early 21st century Earth and are shocked to discover that the humans of this era eat other animals. After the credits, they decide to try another future Federation member world...
TASHA: Their live animals were beamed aboard. We were going to preserve the meat for them, but they say we must bring it to them alive. RIKER: Then do so. Lieutenant Yar was confused. We no longer enslave animals for food purposes. ANTICAN: But we have seen humans eat meat. RIKER: You've seen something as fresh and tasty as meat, but inorganically materialised out of patterns used by our transporters. ANTICAN: This is sickening. It's barbaric.
Future humans will find killing for meat barbaric. Quite a few of us do already, and as "other options" for meat are opening up, it will become more.
Because animals eat other animals. Therefore, these vegan people are bigots against lions, sharks, and the noble T-Rex. Those aren't changing their power. It's not a slight against vegetarianism but it's a slight against the logic presented there. Animal solidarity doesn't exist and any choice on a moral level must come from human uniqueness, not similarity.