This is something I have had all my life! I shall start by saying I decided I was a vegetarian when I was 10 years old, and this lasted for about a year - however during this phase I also decided that McDonald's cheeseburgers and my mum's stew and dumplings were exempt! I eventually realised I wasn't actually vegetarian so just carried on eating all things as normal (to my mum's relief 😅)
The people who assume I am a vegetarian now are people who didn't even know me then - however it has been happening for the last 20 years - mainly at work but not exclusively! I eat meat at work regularly, never really discuss food or dietary requirements with anyone, and I think I eat pretty normally - but it still happens! To the extent that I've started to wonder, is there something particularly vegetarian-looking about me? Or acting?
I ordered the nut roast at a work Christmas meal a few years ago, but that's just because I fancied it! And when I got there it had ran out and I just told them it's ok I'm not actually a vegetarian- I'll just have the turkey.
What would make you assume someone is a vegetarian it they've never told you that they are? I have even been asked if I'm a vegan twice in the last year!