First, I'm really sorry that you don't understand what it is like to have strong convictions toward something (if you had strong convictions toward ANYTHING, you would at least understand, even if you still disagree, which is fine by me). I feel it's a really important part of life to believe in something, to stand strong, and to do what you think is best for yourself, for the earth, for other beings, and for other people.
I think you are missing the point...no one is being fanatical or "militant" here. It's a part of our belief structure.
As for being rude to your host - a good host, would not pressure their guest (in ANY situation) to eat the food that they made. If it was made in violation of someone's religion, ethics, morals or principles, I don't care who you are - you don't expect the person to eat it. A good host would consider this a learning experience to learn more about what they can cook their friend when they come over.
The rejection of the contaiminated food would not minimize the impact of the vegetarian on the earth, you are correct. However, for some, vegetarianism/veganism is not about the impact on the Earth - it's the impact on the animals that concern is about (I actually fall into both camps, with the impact on animals being first for me). As for the excessive impact on the Earth, as a whole, vegetarians and vegans impact the Earth much less than omnivores, therefore the wasting of the food still does not compare to the damage that a omnivore does. This isn't about wasting, it's about principles.
I feel there is a positive effect to any rejection. If the rejection is done in a quiet, polite and civilized manner, such as "I'm sorry, but I'm unable to eat that because of (insert contamination reason here). However, thank you so much for considering my choices when making the meal - I really appreciate it", that opens up the lines of communication of how, what and why the vegetarian/vegan eats. It also allows the host and the other guests to try something different, therefore becoming more educated. I've never been to a dinner party where if the host is aware of my being vegan, I couldn't at least eat a salad or vegetables, so its unlikely that the person would be sitting at the table doing nothing and making everyone uncomfortable. Whenever we can learn from each other, I consider that a positive effect, so I respectfully disagree, and I'm very sorry that you are unwilling to open the lines of communication and learn something about other people.
Acceptance is key and if they feel as though vegetarians/vegans are too "extreme" for their liking, then don't socialize with them. Just because you find them repugnant doesn't mean that the person's beliefs are any less valid than your own.