If I am invited to a meal by someone who does not know I am a vegetarian, I'll eat whatever they serve (unless it's sea food, to which I am allergic).
Generally, people who know I am a vegetarian make sure there is enough side dishes I can eat. That way they can eat their meat and I can eat my veggies.
What gets to me when they serve everyone else a full meal and toss me a plate of garden variety salad. Now, who's being rude here? To them I usually say, "I'm a vegetarian, not a rabbit."