While you ponder that question, let me explain a bit about where I first heard it. I was on a walk around town with my father-in-law, stopping occasionally in a bar to have a glass of wine. The new Japanese restaurant had just opened, and he said, "Hey, why don't we try out the Chinese place?" One thing I've learned from living in Denmark and Spain is that very ethnically homogenous cultures have a very strong sense of "us vs. them" when it comes to immigrants and foreigners in general. In Spain, anyone from Japan, Laos, Korea or Vietnam is considered "Chinese". Keep in mind that, in Spain, even white collar workers will refer to a hard day of work as "trabajando como un negro", i.e. "working like a black man". There's not a lot of political correctness in Iberia...yet. I corrected, "No, it clearly says that it's a Japanese restaurant. Japanese is not the same as Chinese just like Italian isn't the same as Spanish." So we enter, and I'll be damned if it isn't a Chinese family that, with training in Japanese cuisine, has opened a Japanese restaurant in my town. There goes the political correctness lesson out the window.
The family has been in Spain for 5.5 years, the same as I have, and have just moved into town to set up a restaurant. Their Spanish is considerably worse than mine, and my father-in-law's southern accent doesn't help matters. We teach them how to properly pour wine, and quickly discover that the teenage boy is the most fluent. It's at this point that my father-in-law pulls out the joke you read at the top of this post, to a hormone-laden teenage boy. I may have facepalmed.
Like many jokes of this kind, the actual ethnicity involved is unimportant. Any group sufficiently foreign to the teller and receiver works well, as long as it's "other". You can use people from the neighboring town, or rival university, or job title, or whatever. But it takes real chutzpah to tell the joke to an immigrant using their nationality.
So you're with me on the awkwardness of the situation?
The poor lad thinks nervously for a second and shrugs...
A: To put their legs through.
The reason I call it The Perfect Dirty Joke is because it's not dirty at all. It's like a Rorschach test of dirty jokes; it's only as funny as the listener's mind is dirty. As far as dirty jokes go, it's not particularly misogynist, but the gender, like the ethnicity, of the underpants wearer could be reversed with little consequence. The genius is in its simplicity.
The lad laughed politely; his levels of misunderstanding, offense, and embarrassment were hard to judge.
Thus I bestow this gem unto you. Should you have the courage to ask it about a minority to that same minority, then you are braver than I.