Given that we have already had two rape jokes in this thread, you might want to reconsider your notions of what people "can" and "cannot" do here.
Additionally, I need not "tease out a scenario" where it "might be okay" to make a rape joke. I am knee deep in such scenarios (unless you actually believe jokes about rape to be morally wrong, rather than merely distasteful; I know I don't). Less common are scenarios where I can make a rape joke without negative social consequences, but there's still no shortage of those, either.
"the more sensitive the topic under discussion, the more careful you have to be to avoid offense."
You assumption is that one would want to avoid offense. (note that I don't mean 'not cause deliberate offense') Western newspapers were told to not run the Mohammed cartoons, so as to 'not cause offense'. Feel free to defend the position that people shouldn't make rape jokes, but basing that on 'you shouldn't cause offense' is not an argument.
the more sensitive the topic under discussion, the more careful you have to be to avoid offense.