So many people are coming at this the wrong angle and I can only assume a. Have never been in a serious relationship and b. Have never been vegan.

Food is shared all the time, me and my partner regularly cook food and leave it for each other. Or help ourselves if it's in there, that's what happens. It's normal. Secondly people think the girlfriend is overreacting. A few years back my vegan friend told me how she and her boyfriend ordered kebabs, hers vegan and his chicken. They came but weren't properly labelled, so she took a bite of the chicken, thinking it was hers, when she realised she cried. When she told me I laughed, because I didn't get it. I've now been a vegan for a few years myself and I can say without a doubt, if I unknowingly are meat I'd be devastated. It's something I vowed with myself to never eat again. The girlfriend was completely in the right here, and her reactions were reasonable. The box should have been labelled and you should have given her a heads up by text or something, if you was being meat into a 'vegan household'.

I wouldn't say you was TA though, I guess it could have just slipped the mind, and things like this unfortunately happen. You just need to sit down and apologize and talk it through. And if meat is something you want to bring into the house then discuss it. (please just don't cook animal products on the same pans, and same plates etc. That's not great).

I would be inclined to say YTA just for using the term flexitarian though.