Okay, this is a subject that I've become rather touchy on, so forgive me if I seem a bit annoyed in this one.
Too many websites use terms like "Mature Content", "Adult Content", "Not Safe for Work" (NSFW), and other similar ambiguous words to denote content not meant for minors. If you look at the rules on some of these sites, you'll find that they don't even list an age range, just the subject matter those terms are meant to cover, despite intending everything to have a hard-coded age limits. In all honesty, this ambiguity simply makes me cringe from a user experience standpoint.
Why does it make me cringe? Because not all cultures follow the same rules regarding what's appropriate for "minors", and even fewer have the same age of adulthood. In some cultures, an "adult" can be as young as 13 years old, while in others, they have to wait until the age of 21. And even then, in some cultures, sexual imagery is considered "appropriate" for a mature audience while violence or alcohol can be considered adults only. The list of differences goes on, and on, and on.
Because of this, people from other cultures do get confused when an English site--especially a US site--suddenly reprimands them for marking something sexual as mature. In their culture, minors could be more than welcome to see such imagery, so it's not really clear to them without having to dig through the Terms of Service. In some cases, it's the only place that the admins make such policies clear.
From a UX standpoint, this is facepalm-worthy in my eyes. Clarity is something that should be ingrained in a website's design, not added as an afterthought. If a website intends something to be made for a US-18 audience, they should label it as 18+ or US-18+, and explain what that means. Likewise with content meant for a US-13 audience, which should be labeled as such. With those labels and explanations, there is no ambiguity, and therefore, the rules are clear to everyone.
Seeing a great artist from outside of the US becoming so frustrated by an art site for reprimands over adult content that wasn't adult for their culture made me want to slam the site for "UX idiocy". In all honesty, the site in question is run by devs who couldn't recognize good UX if they were using it themselves--and given their number of MacOS-using devs, this is rather apt.
UX is something that most developers shouldn't be taxed with. It should go to dedicated UX/UI people, who can figure out where the problems lie, and what can be done about them. Unfortunately, that's something that sites are crowdsourcing now, which usually ends up making the problems worse. "Rule of the Majority" is not how UX/UI works, because the majority can create mob mentality. It's always best to go with the "Small Groups of Smart People" approach, because those dedicated to the task can see it from more views than just the majority of the users, helping those who aren't part of that majority.
Sorry-not-sorry for the rant, but I really needed to get that out. Dealing with poor UX makes me frustrated, and seeing others suffer from it makes me pissed.