I hate antivegans.
I'm not vegetarian or vegan, but I can't have cows' dairy and tend to avoid wheat (autoimmune diseases that affect joints, tendons, skin and your gut are bastards). I can, however, eat meat and fish that still looks like a lump of animal, goat or ewes' products, eggs, fruit, oats, veg and nuts/pulses/seeds/beans.
I've worked at places where it's been compulsory to attend events/eat what's given and sat in front of tables of pure beige with a cup of water.
I also walked into one restaurant for a do where I was thinking 'Brilliant choice, this is the perfect place to get something I can eat without it affecting anybody else', only to be informed before I'd even sat down that everybody was having a, b and c and if I was about to announce I was vegan, I could fuck off now as In This Team We Eat Meat.
And the number of times people thought they were being clever about 'mmm bacon'. I like bacon a lot. I like crusty bread bacon rolls with butter and ketchup a hell of a lot. But I can't fucking eat them and a strip of bacon by itself just doesn't have the same appeal, so piss off. The lectures about eating disorders and attention seeking weren't particularly welcome, either.
If I go out, a vegan meal means I can just look to see whether there is a significant amount of wheat involved - the rest of the thinking has already been done for me. Same applies where there are requests for dietary requirements - it cuts down the explanations.
In short, if you don't want to or can't eat animal products, you have to tell people or they will automatically assume you eat dairy/generic beige shit or be incredibly unpleasant. At least if you've told them, you know they're deliberately being unpleasant to you, rather than just generally.