My way of handling people who think I'm judging them by being vegetarian is simply to be unobtrusively vegetarian; I cook vegetarian meals for myself, I usually look for a vegetarian alternative, but if all there is is meat, I'll not make extra work by asking someone make me a different meal, nor make things awkward by sitting out.
Of course, this isn't terribly helpful if you refuse outright to eat meat, or if you don't cook most of what you eat, but perhaps those of you who are trying to transition into vegetarianism, it could be a good intermediate stage.
On the subject of nutrition; protein is not even remotely an issue. Wheat is between ten and fifteen percent protein, let alone beans, nuts and dairy. Also, complementary amino acids, whilst probably a good idea to think about anyway, are really quite hard to get wrong. Iron is a much bigger issue, but if you're not trying to bodybuild, it's not too big a deal either.
There are twenty amino acids that make up every protein on the planet. Some of these are produced by the human body, but the rest have to be included in the diet; these ones are called essential amino acids. Any one source or similar category of sources of vegetable protein (e.g. grains, legumes, seeds) will have some but not all of the essential amino acids in large enough quantities to satisfy your body's need. Getting all of the essentials is generally achieved by eating meat (animals need to have them all to live, so they are of course present in animals) or by eating complementary sources of vegetable protein; i.e. grains and legumes, together; you can see this in vegetarian cuisine from all over the world; dhal (lentils) with rice (India), beans with cornbread (Central America), even the humble peanut butter sandwich. In any case, if you're eating enough to fill your belly, you'd be hard pressed not to be getting enough of each essential amino acid.
[/Professor Blarg]
E: Also an actual WTF moment; apparently I'm older than Ein.