Is a vegetarian diet that hard a concept to understand?

I agree Titian1948 a lot of us are unaware of the finer points of vegetarianism, but when your host has tried to accommodate your needs it seems rude to be so rigid in your approach. If you really feel so strongly about your diet then perhaps you should stick to eating with like minded friends. Most of us will try to supply a dish that is suitable for vegetarians but not within such strict parameters.

There was a young man on a programme we saw from Australia that announced he was ' Kangatarian' ….. he only ate Kangaroo meat

I wish all manufacturers would put "suitable for vegetarians "if it is.My DD and her family are strict vegetarians and I have to spend ages checking the ingredient for substances such as rennet

Here is a list of each level of vegetarianism, starting with the strictest.

Vegan: Vegans do not eat anything that comes from animals. This means that besides not eating meat, they will also not consume foods such as dairy, eggs or even honey. Many vegans take this method beyond their diet as well and avoid wearing animal products such a leather and wool.
Lacto Vegetarian: A lacto vegetarian does eat dairy, but does not eat eggs or any other type of meat.
Ovo Vegetarian: An ovo vegetarian does not eat meat or dairy, but does consume eggs.
Lacto-ovo Vegetarian: This is the most common form of vegetarianism. A lacto-ovo vegetarian avoids all meat, but will consume dairy products and eggs.
Pollotarian: A vegetarian that consumes chicken.
Pescatarian: A vegetarian that consumes fish.
Flexitarian: The flexitarian follows a mostly vegetarian diet, but will occasionally eat meat. Many people who prefer the taste of fruits and veggies over meat are flexitarians without realizing it.

I know vegetarians that eat cheese.

Oh dear!
To most people vegetarian means no meat or fish and vegan means nothing from an animal, eggs dairy etc.
You clearly have a very rigid approach, I would be very nervous inviting you for dinner. ?