In the UK, this is already the default: adult content is blocked by ISPs, unless you check a box. The percentage of people who have the box ticked roughly correlates with the percentage of households with children. This bill goes further in requiring age-verification to be run by every site serving adult content, or be blocked in the UK altogether. The reasoning is that children can still browse pornography if they are not connected to their home network, and this will stop that happening.