You can have a "no local pornography" rule without also defederating from instances that have pornography.

The instance I'm on, lemmy.today, has an admin who disallows local pornography communities on the instance and explicitly says that he's fine with users participating in pornography communities on other instances through his instance. From its server rules:

No local pornography communities (but you are free to subscribe to them on other instances if you wish).

That makes sense if you don't have some sort of desire to block it from users on the server, but just don't want to deal with the administrative hassle that comes with it.