That's not in any way unique to online ID checks. If a jurisdiction decides sex education material falls under "adult", then it does, and you have to handle that if you want to serve that jurisdiction. If it doesn't then it doesn't. Same as someone who wants to run a physical bookstore and wants to carry such things. I'm not seeing the complexity. If you don't like the line a jurisdiction draws, the thing to do is complain about that, not say that online businesses can just ignore laws that everyone else has to follow (and sites like imgur are well-resourced businesses with 10s of millions of dollars backing them. They can absolutely be expected to follow laws).
It's also easy for almost everyone to avoid worrying about the lines by just... not trying to exist right along them. If your photography discussion site just has a "no nudity" rule (or blanket puts nudity into its own adult-only section), then you don't have to worry about whether your photo is tasteful art or porn. These are normal rules anyway because sometimes people want to look at their hobby sites in public or at work and just see bird photos or whatever and not have passersby think they're a gooner, or see a surprise decapitation, etc. Even 4chan moderates their hobby boards and separates which ones allow adult material.
It's also easy for almost everyone to avoid worrying about the lines by just... not trying to exist right along them. If your photography discussion site just has a "no nudity" rule (or blanket puts nudity into its own adult-only section), then you don't have to worry about whether your photo is tasteful art or porn. These are normal rules anyway because sometimes people want to look at their hobby sites in public or at work and just see bird photos or whatever and not have passersby think they're a gooner, or see a surprise decapitation, etc. Even 4chan moderates their hobby boards and separates which ones allow adult material.