Orach Chayim 211 is about what food to recite a blessing over (that is, and eat) first when one wishes to eat two foods. It's clear that (except according to the Rambam cited in 211:2) one does not always eat first the food he wishes to eat first. (Mishna B'rura :13 rules that we don't follow that holding of the Rambam's.)
However, the Rama :5 adds an important caveat to the general rule:
And all we've said, that one [food] precedes another, is specifically when he wishes to eat from both of them…. However, if he doesn't wish to eat from both of them, he recites the blessing over [and eats] only the one he wants to eat, even though the other is also placed before him.
It seems obvious (and I believe, though I don't have them handy, that this is clarified in latter-day hilchos b'rachos books) that the Rama's exception applies not only to someone who wants to eat only one food altogether, but also to someone who wants to eat only one food now. That is, if dessert and my main course are both on the table, I need not eat dessert first, even if it should in theory have priority.
These two details are opposing forces in the halacha. On the one hand, one cannot decide what to eat first: halacha dictates what to eat first. On the other, the halacha is overridden by his lack of desire to eat something immediately: in a sense, he can decide what to eat first.
What's the demarcation? To what extent does the Rama's exception hold, and when do we say "that's not enough of a not-wanting-to-eat-it-now to override the general halacha"?