My first reaction is yeah, you don’t just plug into random Ethernet. The wi-fi is likely a visitor network setup for guests to the library. That ethernet port could provide access to their private intranet, and be a security risk to the library. Worst case scenario, it could result in malware, ransomware, and/or millions of dollars in expenses to recover (on a library budget, that could mean permanently shutting down the library even). After reading your post, I would say, no harm intended, just don’t do it again. After reading your comments about intentionally being vague about ‘plugging in’ to lead the librarian to think you were asking to plug in a power cord, and not specifically meaning ethernet connection… yeah, you’re clearly in the wrong. Just be up front; if they say no, so be it. They may be able to direct you to a visitor ethernet plug-in, or maybe not. If this were an AITA thread, i’d say yes, YTA in this case.
Asking in an security community… I would assume some level of technical awareness, and you are likely well aware of network segmentation, and that no IT department would be happy about a guest plugging their laptop into random rj-45 jacks around the building. Maybe it’s not well designed, and that actually has access to firewall administration?