You list some really good reasons why joking about rape is a bad idea. I agree with them.

However...

Hate to be Captain Obvious again

Any chance Captain Logical Consistency is around? I feel like your heart is in the right place, but your logic is alllllll over the place.

[unlike a rape joke, the joke about murdering telemarketers
is okay because of] the reality that telemarketers aren't
being actually murdered en masse (whereas, as #metoo showed,
women are raped en masse)

Sure, I agree.

Gallows humor would be me saying, to another Jewish person,
that we'll going to be debating whether a particular act was
antisemitic or not all the way to the concentration camp.

Agreed.

A sexual assault survivor telling that rape joke would be
gallows humor, too.

OK, so let me get this straight. Only actual, literal sexual assault survivors can joke about it and have their jokes justified as gallows humor?

However, you can joke about concentration camps, and that's gallows humor, despite not having actually been sent to one? As one of Jewish descent, I do not dispute that the reality of the holocaust has affected you in myriad direct and indirect ways that I can not fully understand.[1] But the same could be said of sexual assault. The statistics are stark: essentially, literally everybody has been affected by it directly or indirectly. I haven't been raped, but loved ones have.

And how does this relate to the telemarketers example? You say it's okay to joke about killing them, because telemarketer murder is not really a thing and therefore we recognize it as a joke, however, you can joke about Nazis when Nazis (and their ideological cousins/descendants) are still very much a thing?

In the sense that perpetuating a stupid joke like that
does not mean the person is endorsing it consciously.

But the net effect amounts to the same.

Right, I think that's part of the problem with rape jokes - rape is (incorrectly) seen as a gray area by many, so I would generally say that joking about it is a poor choice. It is also understandably upsetting for some survivors to hear as it dredges up memories of the worst moments of their lives.

But I think you're overlooking some key things here. When examining humor it's crucial to understand not only the topic being joked about, but the target or butt of the joke.

If I say, "so-and-so is a shitty person; he probably rapes gerbils in his spare time" then that is a pretty bad choice. I wouldn't say that. But it also is pretty impossible to frame that as even an implicit or unconscious endorsement of rape. While a poor choice, it can't be compared to a "joke" like "9 out of 10 people enjoy gang rape" or whatever.

Going back to rape, we'd live in a better society if men
were taught by the culture that sexual intercourse with
an unwilling participant is inherently unenjoyable.

Unless I'm completely out of touch consensus is that forcible rape is about power. The rapists aren't under the illusion that what they're doing is enjoyable. The unenjoyability is the point.

Other forms of rape are probably more about a lack of empathy and/or communication. Partner 1 understands that unwilling sex is bad, but has deluded themselves into thinking that sex is really what Partner 2 wants.

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[1] However, my grandfather (Jew) nearly died in a Nazi labor camp, so I'm not entirely disconnected from that reality either... at least we can hate Nazis together.