Well I disagree on the freedom of speech issue, but I can at least see and understand the other view.
Back to the topic, you mention software like Websense, I personally have no experience with this kind of software as I don't have kids but I agree with you that there is excellent software available to parents to implement. The reality is that this cannot be done in an opt-out basis from the ISP.
I don't want to get in to the nitty-gritty of how the internet works, but think of it like the postal service, with the user and the website sending letters to each other. The ISP is like the Royal Mail, it only sees the delivery address and return address, not the content of the message. This proposed system will simply stop letters between porn websites and the user based on the addresses.
User installed software can read these "letters" (known as packets in internet lingo) and filter out content based on keywords. This presents an entirely new set of problems, because keyword blocking in very indiscriminate, for example it often blocks sexual health articles. While this could be partially (but not fully) recreated at the ISP level (although would be an outrageous invasion of privacy and would slow UK broadband exponentially), it would still be trivial to avoid with encryption. An example of this kind of encryption is when you order things with credit cards and the little padlock symbol appears, users don't even notice it.
This kind of system is much more powerful when installed by the end user, and again comes back to parental responsibility and supervision. These systems work pretty well in general, but still have their faults. Simply put, it's not something than can be implemented at the ISP level.