Some sports are banned because of the harms they cause.

https://en.wikipedia.org/wiki/Illegal_sports

I don't think porn is in this category.

I have not seen any real evidence that would justify the concern, and for the most part the indications are the opposite — there is less harm when it's easier to get.

But for the sake of argument, if it can be demonstrated that some content is harmful, then a ban is justifiable.