Smurf said:
I wouldn't consider it rude so much as... well... stupid. If you're a vegetarian you're part of a minority, you should assume that someone doesn't know unless you tell them... and if you don't tell them they'll try to feed you meat. It's... common sense.
So if someone invites a person who is vegetarian, but doesn't know they are vegetarian, and are only informed after they have accepted the invitation, are you expected to change your entire menu for one person? This is the part of vegetarianism that bugs me. If I were invited to a vegetarian's house, I wouldn't make a big fuss about being a meat eater or that I know I'll go home feeling hungry if I am only served vegetables, or that I can't think of a more disgusting food than tofu. If that's what shows up on my plate, it won't kill me, and I politely take a VERY small serving of tofu and manage to eat it so I don't offend my host, and when the party is over, I go home and make a sandwich if I'm still hungry. But, then when the invitation is reversed, it seems at least some vegetarians feel the need to impose their eating habits upon everyone and start insisting upon dishes the host had either no intention to serve, no knowledge of how to prepare, or that just don't go with the rest of the planned menu.
It's your obligation to inform your host if you have food allergies so your host doesn't kill you, but it's rude to expect your host to change the entire menu if the food isn't going to kill you, no matter if you dislike it.
Of course, I suppose the best way to approach it would be at the time you receive the invitation and before you respond by simply inquiring if there will be any vegetarian dishes served because you're vegetarian. At this point, the host can respond by either telling you, "Of course there will be" (whether they really planned to have them or not, they've now agreed they will include something), or "I'm sorry, we were planning on roast lamb as the main course." You can then decide if you are willing to take your chances on the other dishes being served with it, or to decline with the suggestion that perhaps some other time then.
I guess it also makes a difference depending on who is being invited and why. If you're inviting a friend over to celebrate something they've accomplished, and they have special food preferences, of course since they are the guest of honor, you'd want to go out of your way a bit to accommodate them and serve all their favorite foods. But, if instead, there's some other event you're celebrating, and someone else invited other than the guest of honor has such preferences, then it would be more rude of them to expect the menu will be catered to their special requests. Basically, it's the difference between inviting 2 couples who are close friends just because you want to see your friends vs inviting over 20 people to celebrate someone's birthday.