Politics has descended into rhetoric and vote-hunting. Nobody in politics cares that much about consequences of policies compared to whether it sounds good with the voters. Porn is a stable bogeyman in religion and politics, a 'great evil that lurks in the dark shadows of the internet' which must be valiantly fought against. Like the 'negative automatic traits' of clinical psychology, these ideas prevent themselves from being challenged: the reality is ignored, rhetoric prevails, votes get won, and nothing gets fixed. The problem with underage people accessing porn is one of sexual education, or lack thereof. Humans naturally seek sexual enjoyment, if starved of this and offered only a few choice morsels, people can be motivated to work desperately. This effect (akin to the squirrel learning an assault course, as shown in a BBC program called Daylight Robbery (2, part 4/4 on youtube if you are interested)) has probably been beneficial in the past, before the rise of modern marketing. Sexual is used in much of marketing because it works. It works because often pictures of scantily clad young women on adverts are all that a young man will see in their day, and their brain will naturally reward and learn things associated with them (the primitive mate hunting instinct, a relic of our evolutionary past, would never have needed to be adapted to modern marketing).
A more sensible and pragmatic viewpoint is that humans in general have sexual desires and fantasies, often quite strong, that leaving these desires starved and frustrated has the capacity to wreak havoc in somebody's decision making. Rather, modern society need to learn to both satisfy and harness these drives, ensuring acceptable and effective outlets exist for everybody so that there is no need to seek sexual outlets elsewhere. Sexual desire, being short lived, is not a good foundation for a long-term loving relationship or a family, and thus in the modern world these things (sex and relationships) need to be less coupled than they have been in the past. Yes, sex has a major place in relationships, and ensuring drives are satisfied is a responsibility of those in that relationship, but how they are satisfied needs to be far less prescribed than it has been in the past. In addition, if there is a mismatch between desires of those in a couple, there needs to be acceptable options if one or both in a relationship are not to be frustrated (and this frustration can have serious detrimental effects psychologically, both individually and on the relationship itself, if that relationship gets perceived as an obstruction preventing relief of sexual frustrations).
Various forms of sexual entertainment need to be available, people need to understand the basic human needs better, how to use sexual entertainment sensibly, when it is a sensible option, how to avoid addiction-like behaviours, how to prevent obsessions growing to the level of being problematic, and so on. Much of this needs to be taught to children in proper sexual education (rather than the traditional religious ideas of 'tell them it's naughty and not to do it, then hope they work everything out for themselves successfully'). Conservative attitudes to sex were probably a good thing back in their day (a few centuries ago, before the rise of modern science and medicine), but these days they do more harm than good. Appealing to them is an effective means of political point scoring (which is what the 'porn filter' stuff has been about).
That said, porn filters by default is not necessarily a bad idea in itself: parents should have a degree of control with respect to what information and imagery of a sexual nature is available, but this control _must be used wisely_ in the raising of children, and that is what I doubt will be the case. Trying to keep the lid on a Pandoras box that was never closed in the first place is stupid and foolish, yet politically expedient on countries such as the UK and the US. When will we learn?