Giving ISPs the responsibility and incentives to censor a paricular subset of the web is precisely the same architecture that the Chinese Communist Party uses for their "Great Fireall of China". The communists use it to filter news and political information as well as porn — but in neither case is it particularly effective. Users who are either knowledgeable or motivated quickly learn that there are easy ways around these filters.
The absurd Utah proposal has been pushed by the CP80 Foundation, which pedals fanatasies of a world where certain TCP ports (80, for instance) are free of any material that they consider "indecent". The group is fronted by SCO Chairman Ralph Yarro. Yes, that SCO.
The chance that a state or even federal statute could (practically or constitutionally) prevent sexually explicit content from being transmitted through port 80 is approximately zero point zero zero zero percent. The chance that politicians could pass foolish laws that cause needless headaches and court battles for ISPs and users, however, is significantly higher.