Answer:
the marginal benefit of enjoying the dessert outweighs the marginal cost of being too full.
Step-by-step explanation:
At a large dinner, you might not necessarily wan dessert as you compare the benefits of the enjoying the ice cream against the cost/effect of having the ice cream on your stomach.
The marginal benefit here is enjoying the ice cream while the marginal cost is the effect of taking ice cream on your stomach's comfort after a rather large dinner.
In the end, if you will rather enjoy the ice cream than bother about stomach discomfort or bloating, then you can have the ice cream. If otherwise, do not have the ice cream.
Cheers.