Lemmy.today prohibits local pornography communities in its rules (the admin doesn’t want to deal with the headaches they bring) but has explicitly said that it’s fine to use remote communities and also aims to not defederate with any other instances (and has not currently defederated with anyone), so that might also be an option, if you aren’t looking to specifically set up a local pornography community.

goes to look up the sidebar

We dont block any other active instances. We will keep it that way unless it becomes a moderation problem.

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

• No local communities about software related to porn.