Poor etiquette? Sounds more like red flags.
That weird feeling that people occasionally get about someone else, the gut feeling or intuition, is the monkey part of your brain warning you of potential danger.
In terms of personal space, as described in the articles I linked to, it varies greatly from country to country and in some places very close proximity is acceptable. So, yes, it is etiquette, and depending on where a person was raised and thus the local etiquette about acceptable distances would drive their intuition. In America we prefer much larger distances for our personal space than in other countries.
As far as the sexual fetish discussion, some parents may not care in which case it becomes up to the child to decide if they want to expose themselves to this information. Most people I know though, would find it disturbing, thus etiquette would suggest not to do it. And people who see it will have a strong emotional reaction.
I think that the intuition that guided Kat was simply a manifestation of the etiquette of personal space and what is okay to discuss with children. Ethan violated Kats expectation of behavior (etiquette) and she had a strong feeling about it. This is how etiquette is maintained and we remain sort of civilized.
The benefit of such an etiquette is that it prevents an endless discussion of whether what Ethan did hurt the kids in some way. There is obviously no true way to find that out and having etiquette for these situations side steps that question without having to argue. Besides, when it comes to kids, always err on the side of safety, right?
“Etiquette tells one which fork to use. Manners tell one what to do when your neighbor doesn’t.”
