I agree it makes things difficult. I turned pescatarian 4 years ago after being behind a load of pigs being taken to slaughter in a 2 hour traffic jam. Now at Christmas or other family meals I bring my own stuff and just eat the veg. That's the ethical choice and it can be hard for hosters - I used to cook meals for 12 which were easy - a roast or a casserole - yum!
However one of my relatives can't eat chilli because of a medical condition. Some of the GC in the family have serious allergies. Both my DCs (born 80s) will eat anything but have friends who would die if any nut is in what they eat, even carrying epipens.
For all the inconsiderate people up post, it's not a fad if you have a life threatening allergy, or an ingredient which lays you low for a considerable time. No one currently knows what the trigger to the rise in allergies has been over the last 40 years - it could be additives, farming practices and pollution, changing medical advice (eg GMC now think babies should have peanut butter smeared on skin or lips at 6 months), changing foods (when did peanut butter come in?)
Clearly there are some precious people who make everyone's life harder by claiming to have allergies when they don't - not least because they give rise to this sort of thread and get truly allergic people a bad name.
But if you have a family with no allergies, no ethical choices and no food types that precipitate problems, then just count yourself very lucky.
If you do, ask them to bring the food they'd be happy with.
Last weekend, in Rutland, the first statue in Britain of the late Elizabeth II was unveiled.