Some people are shit parents and often it's not even their fault that they're shit parents (e.g. their parents were shit, too, or socioeconomic factors) and they've got no way of knowing they're shit parents as they go along (e.g. their children don't have obvious signs of having shit parents when they're with them and they're better parents than most other people they know). People do need protecting from other people who had shit parents, and it makes sense for the state to do things to protect people from the shitness of their parents.

Banning things for everyone (including consenting adults) is certainly not the most technically feasible, effective or targeted solution, though. E.g. if I were the minister tasked with solving the problem, I'd make sure sex ed classes in school covered everything relevant (they already do, but not effectively enough that it sinks in, so maybe I'd commission a report into why), and legislate that porn sites couldn't promote potentially harmful fetish content without clearly labelling it as such. I'd also not necessarily focus so much on the effects on teenagers, as it's not like people magically become able to maturely deal with things they were totally unable to the day before when they turn eighteen, and there are risks if people don't learn moderation and are exposed to the full brunt of everything all at once. Porn blurs the line between fiction and reality because it contains real nudity etc. so it's understandable that people without any other frame of reference wouldn't intuit which bits were exaggerated.