I might get shit for this, but I think ANYTHING can be funny. Here's a perfect example:
Google "Hitler's house". Within the first page, a bunch of images of a house that looks like Hitler. Then there are the Kitlers, the cats that look like Hitler. Here's one of the most hated men ever, and with making fun of him, we strip him of his power. It's a reaffirms that now, after all the pain he's caused, he's nothing but a joke, a mockery. Which is the last thing he ever wanted to be. Personally, after all he did, I find that extremely satisfying.
Then, we have the issue with rape jokes. Personally, I don't consider what Tosh said to actually be a joke because it really um, wasn't, but that's besides the point. It was incredibly distasteful. I made a rape joke last night, and maybe it was offensive, but I thought it was pretty funny. I made a joke about a tentacle bursting through the wall and the girl on the other side raping it, and the tentacle needing to go to therapy afterward. I've been molested, and I don't personally think it's unhealthy to use humor as a way to get over something. Of course rape isn't funny. It's horrible. Just like how dead babies are horrible. Part of the humor of offensive jokes is laughing at yourself for laughing at something so terrible, and also, realizing that a joke and reality are completely different. If people ONLY made jokes instead of acting out the things that happen in them, the world would be a much better place.
I really don't think this argument is any different to the "do violent movies turn watchers into violent people" and "do shooting games turn players into killers" arguments.
That said, I think when it comes down to it, it's all about the nature of the joke, where it's coming from. But that's true of any joke, rape joke or not.
Once I met a guy in a wheelchair who had the MOST positive attitude about it ever. Even moshed with him (and yes, the wheelchair may have cut me up a bit, but totally worth it). He was FULL of wheelchair jokes. Sometimes, with somethings, you just HAVE to laugh, because what other choice do you have?