pros: they already have erotic books and normal movies, stocking porn movies wouldn't be that different other than medium.
cons: idk something about child safety, and you'd have to make sure people couldn't access the porn videos actually at the library unless you were gonna go full hog and set up masturbation booths or some shit.
idk, i'm leaning on the pro side bc i can't think of a non-puritanical reason to allow erotic literature but NOT video porn.
I will say that pornography probably falls under first amendment rights (or whatever, I'd have to double-check) in that patrons are allowed/free to check those materials out. But it's a "mine-field" for actual acceptance-in-community and privacy/etc. concerns.
(That aside, depending on library policy you CAN view porn in the library, just not in front of others/in a "public" way to where children are able to see it. I've had stories about librarians having to kick serial masturbators out. Don't be that guy/girl.)