The option of “I don’t want to do it” may not be on the table anymore.
For this to be on the table you need to convince all the browser manufacturers to implement it and close the sources so it cannot be undone by the users or forked. And remove all the earlier copies and sources Next you need to ban tools like curl or wget because they can pretend to be browsers. If you want something that can't be removed in reasonable time realistically you have to demand full web drm a-la Google but this means a bunch of older computers needs to be thrown away just because they can't use newer browsers and/or newer OS.
Now that we figured out what needs to be done on the client site - let's talk about server side. You need to convince every porn site out there to perform this check. A few more interesting things to think about - how many porn sites are out there? What to do with those that won't follow your proposal?
In short - from technical point of view if you really want to achieve "enforced age verification" - internet must become a very different, much more closed system.