You need to understand that there is a difference between being against child pornography and being against an ineffective filter.
Lets be real. The filter is open to abuse by the government which intends to keep the list secret. There are billions of websites on the internet, with millions popping up on a daily basis ... you cannot come remotely close to filtering all of these websites.
Furthermore, if it actually worked, it'd be incredibly easy to by pass it. All it takes is a simply proxy - the same method school students are using to bypass filters at school. Besides, most child porn is circulated via secret chatrooms and email, not directly on websites. The filter will only focus on filtering websites because chatrooms, email and peer to peer (limewire etc) are literally impossible to filter.
So instead of dumping tens of millions of dollars into something that will slow down our net while providing practically no tangible results, the government should do something effective. Increasing the funding of the federal police to directly address the problem would be a good start.