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 just want to mention that there are hygiene issues to consider. You don't want to borrow a copy of 'XXX Hard Dicks In Wet Pussies: Watch These Four Women Each Have Sex With A Man Who Is Pretending To Be Their Step-Relative' and then struggle to open the box because it's so semen-encrusted.
Lmao this just made me remember at the library I work at a lady brought back a BDSM guide book very late with the book soaked in mysterious liquid. It's sitting in a bunch bags on a shelf somewhere in our storage. I've never seen it myself and afaik no one has confirmed what the liquid is.