That is part of my idea. I don't think it should be water-tight and not circumventable. My personal opinion is if a 16 yo really wants to watch porn or something, and they put in the effort to circumvent something that is a bit more elaborate than just clicking on "Yes" on a popup... They should be allowed to see it.
But that's just my opinion. And I'm not really concerned with what other instances do. It's enough if it enables me and a few other people to have my instance how I like and invite people to my instance without worrying too much. I mean my own server is also the only one I'm held responsible for. As far as I'm concerned other people can do what they like.
And it's kind of pointless to try. Kids don't need Lemmy or the Fediverse to watch adult content. They can just go to Pornhub and click yes. So I'm already in a position where I don't care about other domains. But I'd like to keep my own Website and Minetest server clean. And also potentially offer some more services and at least do my best to do it right there.