None of this would be a problem, of course, as one can like whatever they want. Indeed, one should eat whatever they want in this metaphor because, well, it's a free country (or a mostly free world or something; this metaphor isn't perfect). But unlike Person A or B or C or yourself, Person D believes you're all ignoring Grape Nuts because you hate people who eat them. There might be some truth to that. You're not overly fond of Grape Nuts eaters. They make far too much noise when they chew and they're constantly going on about how good Grape Nuts are for you...if you'd only stop being stupid by eating all those Cocoa Puffs and Lucky Charms and Mini Wheats and Trix, you'd see the light. So you may not eat Grape Nuts for that reason, or perhaps because you just really hate Grape Nuts (it's an acquired taste, after all). So Person D says, "Fine. I'm going to piss in your cereal so you can't enjoy any of it."
You're understandably shocked by this. Why would someone piss in your cereal just to make a point? Isn't that petty? Isn't that rude? Isn't that kind of the opposite of the purpose of eating cereal? Isn't all this supposed to be about getting some breakfast? More importantly, while you can understand some dislike the love you share for Cocoa Puffs, you at least poured it into your bowl solely because you liked it; indeed, the inventor of Cocoa Puffs shared their own favorite cereals so you could maybe enjoy some Pops or Cinnamon Toast Crunch or Froot Loops, and so on and so forth, because that's what we do in a community: share cereals. But Person D decided to piss in your cereal.
The question is this: How do you get the piss out of your cereal? Maybe you just pour another bowl. Or try to ignore the piss taste in your mouth. Or find a way to siphon out the piss and save your cereal before it's too late. Either way, someone has pissed in your cereal. How you react is up to you.