Sure, fine - we can agree on the hardcore porn. Maybe we can even agree on exposed female nipples?
What about sex education material? What about any content that includes an LGBT person? Because if you think I'm being hyperbolic, read page five of Project 2025.
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.