"Humor has no boundaries—it permeates every social context. Undoubtedly humor and laughing are essential parts of what it is to be human. The pursuit of unraveling what is humor and why it is a central component of our social lives has engaged a wide variety of scholars."
Humorous Communication: Finding a Place for Humor in Communication Research http://teacher.shu.edu.tw/~tyu/CTA.pdf
In order for a rape joke to be "funny" you first have to start with the premise that rape is "wrong". It is the f***ing with this premise that makes it humor. When someone tells a rape joke (or enjoys one) they are actually communicating that they understand this right/wrong nature, contrary to "rape culture". Most cultures, certainly the U.S. one, teaches us that rape is wrong. Paradoxically, rape jokes have the effect of strengthening and increasing awareness of this right/wrong nature. However with social taboos (and the prudes/thought police) being what they are, certain subjects are going to be controversial and are just not worth the fallout to some people. If we factored for the possibility of someone's feelings being hurt when "we" engaged in humor/joking, we'd quickly find that there is nothing that can be discussed without a portion of the population being hurt/offended.
Check out: Humor as an Interpretation of Incongruity
“A situation is invariably comic when it belongs simultaneously to two altogether independent series of events and is capable of being interpreted in two entirely different meanings at the same time” In the case of rape, right/wrong.
In the spirit of this post I will tell a rape joke to communicate to others that I understand rape is WRONG. Don't click the spoiler if you think you might be offended.