That's simply wrong -- a single egregious instance can certainly constitute harassment, both in terms of common sense and in terms of legal consequences. What matters is severity -- and though repeating the behavior makes it more likely to be judged as severe, one bad incident can easily exceed the threshold. A quid-pro-quo proposal of sex-for-promotion is a perfect example.
Unless my understanding of the word "multiple" (and "persistently" and "continually") is completely wrong...
https://en.oxforddictionaries.com/definition/harassment
> Aggressive pressure or intimidation. ‘they face daily harassment and assault on the streets’
English doesn't have a definitive dictionary, anyway -- usage determines meaning. It has been the case for decades that a single quid pro quo proposal could be ruled as "harassment" in court. (As it should be!) Is it any wonder that colloquial understanding would reflect legal reality?
A single demand for quid pro quo sexual favors in exchange for professional rewards is a sexual offence, a bad one, and alone it is not harassment. IANAL, though the dictionary definition sticks with me, especially the key words: "multiple", "unwanted", and "intentional".
That's why "harassment" as a word exists, it is to indicate that multiple unwanted events have happened intentionally.
Maybe the legal definition of "harassment" differs from the dictionary definition. If so, a new word for single event harassment needs to be chosen.