The problem is now the ISP has a list of people who want access unblocked to porn. Its a straight up privacy issue where the Government is forcing its morals on the majority and making a list of those who don't agree. It'll shame some people into getting it blocked because they don't want to call up and ask or discuss the change with the other half.
The majority of users don't know of, or aren't away of proxy servers, and it wouldn't be a major suprise if the known ones start getting blocks themselves at some point in the future. Afterall if adults can use them, so can kids, and you need to think of the childen.
Finally, the vast majority of those polled (including a majority with childern) are against the idea. That in itself is more than enough reason for the legislation to be a problem. Considering one must only change it from opt-out to opt-in to make all the concerns go away, I don't see the problem with doing that.