In general, I find analogies to be excellent ways to advocate for veganism. They provide a simple, straightforward perspective on a topic by relating it to a different one — usually one the listener or reader is already familiar with. For example, my axiom that if it’s wrong to do to a human, it’s wrong to do to an animal latches onto the reader’s intuition that there are misdeeds one can commit against a human and relates them to the misdeeds against animals that I am trying to get them to recognize as unethical. Another example I use to demonstrate the irrationality of zero-sum speciesism (the idea that humans are superior and thus can do whatever we like to other species): Believing the Boston Red Sox to be better than the New York Yankees in no way justifies cannibalizing latter’s third baseman.
Another analogy we could use relates to the wearing of fur. Many people don’t wear fur anymore, recognizing the inherent cruelty — animals are often skinned alive — and lack of necessity — nobody has frozen to death because of a lack of a fur coat per se.
Recognizing these facts is far closer to accepting the merits of vegetarianism and veganism than people typically realize. Meat is, at least at present, inherently cruel: no matter how “humane” the process is, someone’s life was artificially shortened for your meal. And of course, meat is unnecessary: vegans and vegetarians exist.
So ask your interlocutor: why is it wrong to wear fur, but perfectly fine to eat meat?
As long as they recognize the problems with wearing fur, they’ll have trouble avoiding acknowledging the same for eating meat. Sure, they may say that meat isn’t inherently cruel, mumbling something about “humane local farms,” but remind them that meat means someone dies sooner than they should. And yeah, they might grovel a bit about meat being “necessary,” grumbling a bit about protein and veganism not being “optimal,” but remind them that saying meat is not necessary is different from arguing that abstaining from meat is characteristic of a perfect diet.
If they’re engaging in good faith, it will at the very least make them think. But if they’re operating in bad faith, they’ll probably throw at you some cringey whataboutisms. Sometimes when you back an omnivore into a corner, they’ll spew some whoppers, like “I’d gladly eat dog” that every onlooker recognizes as dishonest. That’s fine — people operating in bad faith aren’t worth your time, and the whoppers speak for themselves.
At the end of the day, if someone is actually interested in making the world a better place and minimizing their contribution to cruelty, relating meat eating to fur-wearing can be really effective — as long as they already recognize that wearing fur is wrong. Give it a try and see what happens.