You are supposed to for sure. All your followers are connecting to your inbox relays to download your replies. If you add a porn relay there, all your followers will get porn.
That's the outbox model in a nutshell.
Discussion
* download replies to your posts.
that means outbox model is not good with spam/nsfw ?
It's only as good as the author wants.
Its the duty of each author to protect their followers from porn.