Also, while excessively picky eaters who just don't like a whole load of stuff do get right on my tits and I can't pretend otherwise, I think lots of people do get very sneery and like to deride others attempts to improve their health or their weight by eliminating certain things because of possible food intolerances.
What I know happens is that if someone says they are trying to avoid certain foods for a while because of a genuine concern that those things have a negative effect on their health or their ability to lose weight, someone somewhere will pipe up 'oh how ridiculous - she's decided she's allergic to wheat/dairy/whatever now.'
No, she hasn't.
She's probably never said she's allergic to anything but she has said she might be intolerant, which is totally different. And she feels better, less bloated, less phlegmy, less achey and tired, less brain foggy, she sleeps better, she has better skin etc, her IBS or her PCOS or her PMS or her insomnia is more under control, and she loses excess weight by not eating those things. And if she doesn't want to eat or drink something just to make everyone else feel better then she shouldn't have to, and she shouldn't be made to feel really awkward about it.
Having said that, I do hate it when people on very limited diets expect everyone else to jump through ridiculous hoops in order to facilitate that. Either deal with it for a day or two, especially if you know it won't kill you, politely leave what you don't want to eat, or just eat a little of it to show willing, or take your own food, or don't go.
But unless your guest is being really demanding about what they want you supply and cook for them, it's pretty rude to keep drawing attention to that fact that they don't want to eat the same as you, and to make them feel bad about it. It's not that hard to shove something fairly basic and easy that is bland or low carb or dairy free or whatever in the direction of said person, while you carry on as normal, is it?