Raisins? T9ism or a slang term as opposed to an ordinary term?
This: http://www.catb.org/jargon/html/H/hysterical-reasons.html . Although, the sort of historical reason I blame is this context is pure inertia, not backwards-compatibility issues.
Anyway, I mean, you're right, but they kind of have a point. One word sees a much wider use (e.g. as a pejorative) while the other is used primarily in clinical and erotic contexts.
My similar but distinct guess is that it has to do with how the penis has excretory and reproductive functions but the ****oris doesn't: it produces erotic sensation and nothing else.
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One man's converging evidence is another man's pilpul.