For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some 'organic element' since I couldn't accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Also: mushrooms are genetically closer to animals than plants. If and how that impacts vegetarians and vegans, I'll leave to you, the reader.
Mushrooms are plants made out of meat. They are the opposite of shellfish which are plants made out of meat.