Are you, personally, harmed by someone watching porn?
Maybe it makes society as a whole weaker compared to other societies but that doesn't mean your society actually loses anything. Countries have nuclear weapons and multi-national defense pacts now. Just because China has several times the population of the US and could win a conventional war, doesn't mean it's gonna invade.
We don't live in tribes of a few hundred people who are constantly on the brink of starvation and/or genocide anymore. But many people are still governed by instincts from that environment.
And the difficult thing about that is that externalities do exist for some things. Pollution affects everyone, for instance, and those externalities need to be accounted for. It's not a trivial definitional problem to distinguish valid externalities from spurious/invalid claims, unfortunately.
These people are on one end of the authoritarian spectrum. The other are anarchists, usually ancaps in places I frequent, and they are just as wrong because they are ignoring how people in the real world will actually behave. Negative externalities are precisely the thing that they ignore, minimize, or try to redirect your attention by linking you to a 2 hour video which is supposed to address it but doesn't.
Well, at least they do generally get out of other people's lives though. But it's a system which doesn't work and when it fails, leads to the rise of authoritarians. I wish people were able to understand that policy is a multi-dimensional spectrum and extremes on any one axis are unlikely to lead to stability.
A better example would be to consider that pre-op transgender people, namely MtF, can share the same spaces that were reserved to biological women. For instance in sport. If I'm a woman and now I have to compete with a biological man in MMA because society chose that it's the new normal, yes, I'm going to suffer from it.
If society decides that it's fair to feed Tiktok to toddlers who end up having their neurons fried, I'm going to suffer from it as well when they grew up as they won't be able to do their job well. If society decides that it's fair to allow CSAM everywhere, I guess that a few children are going to suffer as it becomes mainstream.
We don't live in a vacuum.
I cannot agree with the toddlers example: 1) it's possible it does some damage but as I said, to even consider it, we need proof; 2) even if they are bad at life, it doesn't automatically harm another individual - don't hire incompetents and they won't harm your business. They might harm society in general but society does not IMO have any right to force people to optimize for society's goals. The balance might change if this was true and happened on such a scale that society measurably declined but then I am still of the opinion that we should have mechanisms how a group of people should be able to get together separate themselves from others. We have no right to force the Amish to start using modern tech, even if a few more able bodies would probably be beneficial on some scale. Similarly, if the majority feed TikTok to their toddlers and a minority does not, they should be able and allowed to form their own society with its own rules, as long as everyone participates willingly.
According to your thinking, no one can "force" medical faculties to say that now anyone can buy a surgical doctor degree for 10$, that said if you have to get urgent surgery you'll pay for the consequences as the "doctor" who'll operate won't be qualified.
This is why we need basic rules, norms and constraints that allows us to reach a better optimum than the free-for-all that you recommend. It's also a question of personal values: I value more living in a CSAM-free society, than one where pedos can watch it freely. Amishes value being in a highly religious and low-tech society than giving birth without pain or having air conditioning.
I agree that leaving the society should be possible (emigration, for instance, as the Amishes did). But each society has its local optimum, so you'll have to choose with different constraints. You can for instance go to Russia where it's legal to own and watch CSAM, but with other constraints.
