First of all, the logistics of managing and maintaining what is allowed and not allowed by each ISP subscriber is a monumental task.
Second, the task of deciding what is porn, and what is not porn would be a thankless job (or a fantastic job

). But it would take tremendous amount of cost and infrastructure to build these 'gatekeepers'.
Plus, you would then need an enforcement mechanism, which may not be practical or effective if the violators are in other countries.
To make this work there would need to be standards and ratings (think MPAA PG, PG-13, R, etc), however HOW do you then apply those to EVERY image, movie, or written material on the Internet??
Is the victoria's secret catalog porn? What about medical textbooks showing human anatomy? Internet chat rooms can be sometimes totally tame, and other times quite inappropriate...all depends on the users; that's not possible to regulate. It's simple to create such a law, but applying it would be virtually impossible, and circumventing it would most likely be trivial.
Finally, there is the practical issue that multiple people in a household use the same Internet connection. There is no practical way to let a parent have access to any material while blocking what children can access.