> I think
We should probably actually attempt to quantify this in more than just a feeling before talking about legislation.
> having children and teens exposed to limitless hardcore pornography is likely harmful to them.
Like above, the fact that you had to use the word "likely" means it is an argument of feeling not fact.
Which you do admit in your second sentence.
> benefits to society of allowing this are essentially zero and the harms likely severe.
I would argue the opposite. There is a major benefit to society to allow people to learn about themselves and their sexuality in a safe manner through exploring self pleasure and porn.
And then as a society de-stigmatizing sex and porn so we can have open conversations about it. So If I was 13 or 14 I could go to my parents and ask about something I saw in a video, something that may or may not be "normal". Maybe it was a BDSM scene and it felt weird that someone was "hitting" the other person. That leads into a conversation about consent and communicating what it is you do and do not want out of a sexual encounter.
That de-stigmatization also leads to discussions about what a real body looks like, how sometimes things may hurt and you may need to ask to slow down or stop. All of the unsexy things that are not shown in porn, that isn't a problem with the porn. It is meant to just be sexy. But we could talk about it.
We only think of this as abnormal because it's sexual related, but it's completely normal with video games, movies, whatever. To see something weird and to ask friends or family about it. Hopefully sparking a discussion.
Sex is a perfectly normal thing for most people and yet we treat it as this dirty secret that we shouldn't talk about.