Let me re-iterate, as no-one seems to have understood my last post. I don't think a letter granting consent in the future can carry any weight, for precisely the same reason that a letter of non-consent wouldn't.
A letter might matter if there was no communication at the time. I'm assuming that is an extremely rare scenario.
You've missed an element. Have another look at the OP. It goes: do not want to have sex with them unless and until further formal notice from me?
This is a satirical thread, designed to send up the social attitudes of rape apologists, who behave as if lack of consent has to be proven, not consent. I have seen some truly horrific threads on the internet, after rape victims asked for support with clear-cut stranger rape, where they were met with 20 pages of vehement flaming for considering "ruining an innocent man's life". They were asked how the stranger could have known they weren't consenting, if they remembered feeling unable to struggle.
This hypothetical letter is about that attitude. It's not a sexist thread demonising men as rapists. It's NOT aimed at telling men not to rape, because 19/20 know not to, and the 20th either only fears getting caught, or doesn't acknowledge that he's a rapist. So he'll ignore it, too.
This is about saying, "so at what point would rape apologists hold up their hands and say, 'yeah, Josephine Smith isn't at fault for not saying no clearly enough. The rapist was responsible for checking she was actually consenting right then. He* had absolutely no business presuming consent."
What would it take? Absolutely unworkable measures, like a letter to 3.5 billion people, just on the offchance one of them is a rapist who will one day target you?
Amanda is exposing society's attitude to consent for the illogical absurd mess it is. A mess that victimises rape victims further, by seeking any excuse to deny their experiences.
*I use 'he' because this particular social attitude is, IME, an attitude towards female victims of male rapists. Male and female victims of female rapists are also ill-treated by society at large, but in different ways.