I'm not from your instance, so take my opinion with a grain of salt, but I specifically looked for an instance that could access all the communities I want to take part in. You probably have users like that on your instance too, who would be sad or would even move to another instance if this were done after they joined.

In your settings, you can choose not to see any NSFW content. Similarly, when you're not logged in on an instance, any NSFW content is hidden by default. While registering, it asks whether you want to see that, so I think you must've picked yes.

The main reason to defederate from instances with NSFW communities, as far as I'm aware, is that your instance will cache anything one of its users views from another instance. If that thing includes stuff that's illegal in the instance's jurisdiction, that's a problem... Lemmy doesn't even cache images, just the text, but that would contain a link to whatever content's in the original post.