That's true, but most Lemmy clients default their sort to "Local" (only posts from the instance you registered on). I don't think there are many dedicated NSFW instances that are federated, at least not with my instance (lemmy.ml). Your experience may differ. The problem is that if few people can access and are aware of a community, then fewer people will post in it. That might explain why there is a lack of good porn communities on Lemmy compared to Reddit.