Darvin: The prospect of institutionalized cannibalism is so preposterous as to be impossible. It gets a pass precisely because it couldn't possibly happen. If cannibalism of the infants of ethnic minorities had actually been a thing that happened in contemporary British society at the time, then the context and meaning of the work would be completely different.
When you joke about rape, however, you're talking about something that actually does happen to people in our contemporary society. There really are men out there who do rape women, given the opportunity. These aren't some sort of mythical trolls lurking beneath bridges; they're real people, often with no criminal records, who act completely normal until they find themselves in a situation where they are alone with a woman. There is really nothing to distinguish between a "joke" and an actual threat in this regard, no way to distinguish between someone who just thinks it's funny and someone who seriously does believe that raping people is "fun" and would actually do so given the chance. When you make jokes like these, you're associating yourself with real-world sickos, and it's sad how this fact is lost on so many people.
Well actually that's not true. There were several people who took Jonathan Swift seriously at the time.
Although they obviously didn't agree about the eating children part, they did agree that removing Irish children from their parents if they were too much of a burden was a good idea...
Furthermore, the point of that essay was that given what the English were doing in Ireland at the time, in Swift's opinion eating Irish children would have been a logical follow-up.