I know there is an abundance of feminist blogs addressing this issue, but I couldn't resist putting my two cents in.
Rape jokes can be funny. Example
The question often brought up is "why can we joke about death and not rape"
It is interesting to deconstruct. Perhaps it is because death happens to everyone. It isn't insulting one group of people. Perhaps also the fact that real rape is often a target for joke comes into play as well.
However there are obviously bans on certain type of jokes. Torture jokes don't seem to be common.
The joke is "rape is just surprise sex!" not "torture is just surprise bdsm!"
It just doesn't have the same oomph because people recognise torture for what it is; wrong.
However in a culture where rape is often considered to be rough sex, it hits a little too close too home when people minimise it by referring to it as sex. This misconception damages rape victims, women and society. Why perpetuate the myth?