Running a CNC machine is definitely about sex and should be demonetized

Maybe I should have been more explicit: My point wasn't that you implied somebody was wrong. My point was that you implied almost everybody was wrong, and, more importantly, did not elaborate. Of course the act of disagreeing involves implying other people are wrong, but if you only assert that they are wrong and don't say anything else, it is insulting/dismissive. In my comment, I gave background for my opinion.

>I don't see anything illicit with her showing the side of her breast

This is a pretty heavily reductive description. Again - context is everything. Even full blown nudity can be non-"adult" for some reasonable definitions of the word (though it's also arguably reasonable to have a definition that does always include full nudity, regardless of cultural disagreement! Maybe we should encourage content hosts to split the word up into more parts, like TV ratings do - what's the overhead on that?).

>does your definition extend to the classics?

Why not? Some of them are quite adult. The Venus de Milo? No. Caravaggio's Holofernes? Maybe? But this is just my opinion on a word: "Adult". I don't have a strong opinion on the implications without more context. E.g. the artists of thematically adult works shouldn't be punished, and children shouldn't be disallowed from ever seeing them. Regardless, as in many cases of analogy, the similarities are only on the surface, and the many subtle and contextual differences cast doubt on its usefulness. The most salient problem with the question is that "how much body is exposed" is only one small parameter that by itself can't determine adult-ness.

Really though, the most important part isn't what makes semantic sense to call "adult", but what the practical consequences are of tagging - if YouTube tags the Caravaggio as "adult", do some advertisers pull out of that painting? What's the likely loss of income for Caravaggio? How much of that is reasonable based on the advertisers' target audiences? How much of it is because the advertisers don't want their product associated with that painting, vs how much of it is simply because advertisers don't think their audience is actually even looking at that painting? How much of the unreasonable portion of consequences is YouTube's fault, and how much is the fault of the advertisers? It seems like there's more interest in tilting at puritan windmills than considering the practical issues, which is really the only point I was making in my original post.

That's not my experience. Again, criticize advertisers for taking ads away from such tame content all you want, or YouTube for making it even possible - I agree (mostly with the former). But it seems pointless to try to make a semantic argument that excludes her content from any reasonable definition of the word "adult".

>in general, the term 'adult' is used (... in the US) to refer to pornographic content

As used in the US, the term "adult" can mean porn, but can also refer to sexual content much more broadly. It's contextual.

>That's what youtube means, it's what advertisers mean

I can see the word "pornographic" being used in the most generic, clinical sense possible to describe any kind of content intended to sexually stimulate, in which case, yes, her channel has pornographic content, but you're obviously using it more colloquially, in the more normal sense that would exclude her channel's content, but then of course that's demonstrably not what they're referring to.

This all seems moot. They explicitly state their definition of "adult", and it is far from porn, and her channel is far from porn, so... what does porn have to do with it?