1) It is a guest's responsibility to inform the host of any dietary requirements. An explanation is not required, but is helpful.
2) It is a host's responsibility to meet a guest's dietary needs, or say if it is not possible to do so. Other arrangements may be made if necessary.
3) It is not okay to pester someone else about their dietary needs or choices.
4) Not every dish on the table has to be something that everyone can eat.
5) There must be enough food on the table, in varieties acceptable to the diners, that everyone will enjoy the meal and fill up on what is there.
One that is not a rule, but a gradually increasing tendency: As I get less and less pleased by commercial foods, I am slowly buying more things that are just raw ingredients to cook with. It is a great deal easier to make things that are edible for everyone that way, and the taste and nutrition tend to be loads better too. The main limitations to this trend are time and expense. It takes more time to make things from scratch, although I'm finding some handy shortcuts. Plus organic/grass-fed/free-range/etc. food tends to cost a lot more than factory junk. Sometimes, though, the good stuff is cheaper. I'm getting better at taking advantage of such bargains.