Do you think it's appropriate to eat food from someone else's boyfriends plate

I was at a dinner with my boyfriend and his work colleagues. The meals were alternated so we were taking bits and pieces of each other's plate. He doesn't like mushrooms so I chopped off half of his and ate it. Anyway he finished his meal well before me but still left the half mushroom on his plate. Then one of his work colleagues (a girl) asked if he was going to finish his mushroom he said no he doesn't like them so then she took it off his plate and ate it. Now I know it might seem like I'm over reacting. But that mushroom was mine. He's my boyfriend I kinda get the leftover food privileges not her. I don't even know why she thought it would be okay to do that.