I'm not sure there's a rule that you have to take some of everything that's offered. It's polite to take as much as you will eat, but no more (or not very much more).
Most adults, I think, have some food or other that they would rather not eat. Some people don't eat certain foods because of religious reasons, or health reasons, or moral/ethical reasons.
So a polite guests takes what they can eat, in reasonable amounts (i.e. no one person grabs all the steak, or all the bacon, or fishes all the strawberries out of the fruit salad). If you end up with something you can't eat on your plate, you don't eat it. One school of thought says that you mess it around on your plate a little, to make it look as though you have eaten some of it.
I can't eat eggs. Not allergic, just the taste, the smell, the texture--they are all repellant. So I avoid them at breakfasts and brunches and load up on the foods I really like.