This is a question of ettiquette rather than ethics. Should a guest accede to the host's norms, or should the host make an effort to accommodate the guests? Most hosts will do the latter, and will do their best to provide food that suits their guests, whether vegetarian, kosher, gluten-free etc. Most guests will do their best to eat what is put in front of them.

I think the underlying complaint is that vegetarians usually seem expect their host to accommodate their dietary preferences, but won't do the same when they are the host themselves. Sauce for the goose, if that isn't an unfortunate metaphor. Where this argument fails is that few people, if any, are total carnivores, and most will also eat vegetables.

I'm one who regards most vegetables as mere garnish, and my heart usually sinks when I'm told to expect vegetarian fare. It usually turns out to be utterly delicious.

While I have some sympathy with the idea that vegetarian hosts should cater for their carnivore guests, it is actually no hardship to have one
meal without meat. Acknowledging the host's sensibilities should be part of being a good guest. My advice is to go to the party and enjoy the food you're offered. If you really can't cope without meat get a burger on the way home.