Straight-up genitals. This probably should not be allowed to happen even when a user isn’t doing NSFW flagging properly and/or an instance is not enforcing it.
You can just search “lemmynsfw” in magazine/community search. It finds 8 pages of @lemmynsfw.com communities for me. And subscribe or block from there.
The downside to that is that that will only find communities from that instance which at least one person on your instance is subscribed to.
For example, until I went to the (general, not magazine/community) search field, entered theonion@midwest.social and subscribed, searching for midwest.social in the magazine/community list would not have shown (on kglitch.social). Now, because I subscribed to it, kglitch.social has it in its list of magazines/communities and people can find theonion@midwest.social in that list.