The joke isn't to "promote rape", the joke is the absurdity of putting a statement like that in a context where it doesn't really fit. In this case the "keep calm" meme.
See: Prison rape jokes.
For 2008, for example, the government had previously tallied 935 confirmed instances of sexual abuse. After asking around, and performing some calculations, the Justice Department came up with a new number: 216,000. That’s 216,000 victims, not instances. These victims are often assaulted multiple times over the course of the year. The Justice Department now seems to be saying that prison rape accounted for the majority of all rapes committed in the US in 2008, likely making the United States the first country in the history of the world to count more rapes for men than for women.
> There is a 7% probability of being raped within one month of entering youth jail
And to spice it up with sexism, the state's minister of justice explained:
> Well, a jail is not a girls' guesthouse.
[1] http://www.tagesspiegel.de/weltspiegel/mobbing-vergewaltigun...
So a joke is offensive, big deal... Be offended, it's your right. Just stop whining about it.
I love this mentality of "I have every right to say something offensive, but shut up if you call me out on it!" It makes it easy to identify the immature ones in these discussions.
For example, in thailand everyone holds the king in very high esteem. So if you were to be in thailand and make a joke about the king, even if it was extremely lighthearted and not very derogatory a thai person would likely take this as a much greater personal insult than something like a rape joke.
In other words, yeah, 95% of guys who hear you joking about rape know that you're obviously just joking. But it turns out that there's one guy who doesn't: he thinks you and the guys who laugh are giving him a quiet nod of support. And you don't know which one he is.
Most clinically-depressed people think everyone will feel just as negatively in response to anything they say/do as they themselves do. They are incorrect about this. The world is not this way; their perception is distorted.
For that matter, most zoophiles think everyone has secret thoughts about how sexy animals are; same for pedophiles with children. They are, obviously, incorrect about this. The world is not this way; their perceptions are distorted.
Mental illness can warp your view of the world, and of what is "predictable behavior" in other people. What evidence is there that rapists are rationally observing a rape-normalizing society and concluding that rape is okay, vs. that rapists are observing any society through a distorted lens and concluding that that society is okay with rape?
Which is to say, more practically: what evidence is there that, in a perfectly "cleaned up" society where nobody ever makes a rape joke, there would be any less rape, or a single less rapist who thought society approved of rape?
If people already are one way [e.g. "making rape jokes"], you should find a factual, consequentialist reason that making them some other way ["not making rape jokes"] would improve society, before going through all the effort to convince everyone to change their collective minds. Without the ends, why the means?
That being said, here's a link: http://psp.sagepub.com/content/32/3/286.abstract At least from the abstract, it sounds like it's addressing exactly the question you've asked (and that previous studies have found related results). (I took that link from a recent blog post by Jim Hines, which includes quite a few others worth reading, too: http://www.jimchines.com/2013/03/preventing-rape/ .)
The main lesson I see here is that the people behind this shirt company set themselves up to sell a horrifically inappropriate item. They weren't being deliberately awful (as far as we know), but they were painfully thoughtless about where their algorithms would eventually take them. And doing harm unintentionally still counts as doing harm.
You could certainly see a lot of humour as potentially offensive to some people, or in some cases slanderous (which I think is worse than merely being offensive). You could also think that somebody who chooses to wear a shirt like this might be advertising themselves as bit of a dick.
Inappropriate is also a sticky concept, it might be an inappropriate item for a kindergarten teacher to wear to work but it might not be so inappropriate to wear around adults who would understand it as humour.