This is precisely why it's nothing more than a piece of fraudulent, lazy politicking, which dishonestly purports to be something it is not. The U-18 pornography age verification farce is exactly the same.
There is no intention of holding teens or Guardians responsible for their own use, instead, some unenforceable legislation that purports to shift the onus onto SM companies to regulate who can and can not use their apps. This presents a number of irreconcilable issues which render the legislation wholly pointless.
First of all, any company not based entirely in the UK can simply ignore the legislation because Ofcom is utterly powerless outside the UK, as 4Chan are handily proving, to the extent that even their lawyers are openly trolling Ofcom's continued impotence.
Then there is the simple fact that by shifting onus to the SM companies, they can simply put up a few cursory restrictions on access from the UK, claim "job done", meanwhile every teen just continues on with SM usage as before because the workaround involves two more clicks on a keyboard or activating an in-browser VPN.
Yet another of the endless examples of UK politicians being wholly incapable of grasping what the Internet actually is, or at least, relying on the public's general ignorance of the fact that UK laws have no jurisdiction whatsoever over Internet entities not based in the UK. The fact their products can be accessed and utilised by people inside the UK is inconsequential.