Earlier quoted context omitted.
Why do you think that explanation is particularly probable out of all of the other possibilities? It seems more likely that society would consider actions strange if the actions have justifications that most people don't understand or agree with, and they don't succeed in masking their judgement.
I think this because there are plenty of vegetarians and non-smokers, and non-drinkers around that people overall don't really find it weird. I myself do eat meat and drink alcohol but it is so little overall in a year that you could almost count me as a vegetarian and as a non-alcohol drinker. I do not smoke at all. Also, I've noticed that people that claim to be vegetarians do it in such a way that they come off sm…
The whole "I'm a vegan!" is similar to "I'm a Christian", "I'm an environmentalist!", "I'm an Objectivist!", "I'm an Atheist!", "I'm gay!" phrases I often hear or read, and that social signaling can easily come off as smug/arrogant/annoying/pushy, and in general just make us not like the person. More action-oriented phrasings like "I don't smoke", "I don't eat meat", "I don't believe in any deity" seem more reasonable and I think they produce better conversation.