I think from a legal point of view, you could possibly toe a line and go by ‘good faith’ etc. etc. I absolutely would not want to be the one on the hook for any edge cases as an instance host/admin, not in a million years.
But morally I struggle to see a strong argument for not taking a strict line. There are enough adult characters to fantasise about if that’s your thing. Leave the ambiguous shit for the dark corners of the web.
I suspect that any attempt to try and find a compromise here will lead to the instance being quite rightly defederated so that users of other instances don’t have to have apparently underage sexual imagery turning up when they view the ‘All’ tab. I can deal with porn turning up in my feed if I’ve ticked the ‘Show NSFW’ box. But seeing drawn CSAM (that’s what it is, brass tacks) is gross.
Oh I dunno, I spend too much on HDDs 😅