It's a tricky one. I would support the idea of making porn less accessible particularly to children. If ISPs required customers to "opt in," this might help achieve this. But, it's never going to be the magic pill.
As fridakahlo says, images and messages from porn, including very hard core and violent porn, are reflected in many facets of our society from advertising to fashion, from music videos to social networking. Filters won't stop that.
Also, even if one family have a filter on their internet, there will be friends whose parents aren't so vigilant. Kids are more tech savvy than most of their parents, so they could find ways round it. And, what about mobile phones and tablets that use 3G or wireless technology?
I think the Daily Mail have backed this campaign because it gives them a chance to get on their moral high horses, present the problem and solution as simple and get all enraged when anyone disagrees with their campaign. It all sells papers, or more importantly, advertising because it generates higher circulation.
The solution, however, is much more complex than this and the problem is now so inextricably interwoven with our society, popular culture and values that it's going to take one helluva mult-pronged approach to reduce the damage (if that's even possible.)