I think your confused by reading that paragraph. Nobody is denying access to the website as you can 'opt-in' to access it. You'd probably be 'denyed' access for a total of 10s it takes to log into your acccount at your-isp.com or by telephoning your-isp-phone-number and giving your info.
This is pretty much akin to the 'Enter/Leave' button already existing on adult websites but with a internet-wide enter/leave button where you can avoid all adult content with one 'leave' button to protect your young 8-year-olds from looking up pornography online.
This idea would be great for business/schools to be able to turn-off porn-access to your Internet connection. It would safe administration costs for IT departments to an extent.
They could just as easily make it opt-in and I'm sure you know most schools and businesses already have filters/proxies in place for this type of thing. Even if this was adopted they'd still have to run proxy/filtering services because there's more than just porn out there that schools and businesses don't want people accesses during school/business hours.
This feels like a non-issue personally. There are plenty of established ways to filter this type of stuff at the end-user level. Every windows PC going back to XP has some sort of parental or content controls that can be setup to block just about anything.
I'm not disputing the convenience of having the ISP handle it for you however, just that it should be opt-in, not opt-out.