Actually, I'd interpret this very differently - inviting someone back for coffee is, on the face of it, saying that the reason you are inviting them is for coffee, not sex. Its a false pretext.
It's a pretext, sure. That's the point. The standard getting-to-know-you script does not allow for directly asking someone for sex (unless you're already screwing them on the regular; "wanna get some ice cream and fuck?" is acceptable, if a little crass, on the tenth date) so we've developed the line as a semi-standardized cover story for getting a couple hours of privacy with someone. You shouldn't read it as "I want coffee", but rather as "I want to be alone with you, so here's a transparent excuse". There are more creative ways to ask the same thing, but because they're more creative (and therefore further outside the standard cultural script), they're more prone to misinterpretation.
Compare the Seventies-era cliche of "wanna come look at my etchings?"
It's a pretext, sure. That's the point.
I think there's a deeper point: human interactions are multilayered and the surface layer does not necessarily carry the most important meaning. The meaning can be -- and often is -- masked by something else which should not be interpreted literally.
"It's a false pretext" is not even wrong -- it's just not a correct way to think about the situation. A "pretext" is a way to express in a socially acceptable fashion a deliberately ambiguous meaning which, if said explicitly aloud, would change the ...
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