Do you want children to watch porn? Do you like it when children see dicks? Why do you want to show your dick to children, Dr. Ow - if that is your real name?
But for reals. I have no problem with internet providers offering a service like this for parents who want to control what their kids watch, but to make it so that you have to opt out instead of opting in is just ridiculous. As much as I try, I just can't understand what goes on in the heads of the people who support this. Why would anyone ever think it's a good idea?
Though I probably just demonstrated in the first paragraph why some politicians decided to hop aboard the Cameron Crazy Train. No matter how good your reasons are for opposing the censoring, some people will always see you as supporting porn and cry out "but think about the children". It's just much safer to support it, just like it's safer to support the War on Drugs.