It's not automatically a given that women don't do things that get labelled as tantrums at a higher rate. To demonstrate this is sexist, you would have to show

It's not automatically a given that women don't do things that get labelled as tantrums at a higher rate. To demonstrate this is sexist, you would have to show that people judge the same behavior as tantrum or not tantrum at a different rate depending on the specified gender. Which may well be the case, in which case please link the study. To turn it around: an interesting self-experiment is to try flipping the pronouns in the story and seeing if your internal reaction to it changes.