You're not really going to be watching porn at the library though, just saying

Age checks certainly won't be restricted only to porn sites. Tons of sites have 13+ ToS like Facebook & the various other social sites, Discord too iirc. The reason people are so adamantly against this proposal is that it ties the machine to a particular identity. So how does a public computer work? If age verification is implemented at the OS level, can we even have public terminals? All those interfaces in stores that are just a website in kiosk mode? Would they be illegally representing end users if it's set as an adult on a master account/login? This proposal is so stupid and poorly thought out it's alarming.

In fact, many libraries have computers sectioned off in semi-private areas exactly for this reason...