There's a difference between making a joke about something, and using something irreverently. I think that a lot of the arguments made against using the word 'rape' are not about the fact that jokes are made *about* rape, but that 'rape' is used as such a throw-away term when people are trying to be funny.
I would argue that nothing should be completely off topic for humor, any more than anything should be off topic for any conversation. But I would say that there are things that we should not desecrate needlessly.
To use the common derogatory usage of the word 'gay' as en example; it's fine to make a joke based around 'gayness', but using 'gay' just as a punchline to mean something bad, is harmfully denigrating the word and those things associated with it.
It's not often I see someone make a joke that is genuinely about rape (as opposed to just using the word rape in a humorous way). I would defend the former, and whilst I wouldn't always take offence at the latter, I do appreciate why a lot of people would, and would refrain from using it. Not just because I don't want to offend people, but because I believe it can be genuinely harmful to trivialise these things through our language.
So to address the original poster, who heard that discussion of rape was harmful "
even when using the word as a generic placeholder for something entirely unrelated", well actually that's exactly why it is harmful.
You wouldn't throw around genocide as a punchline if one had happened next door, so be sensitive to the fact that rape is actually a very common occurrence, and lots of your potential audience has probably had a close encounter with it.