A couple of points:
as has been said, dancing in a suggestive manner is something adults understand- not children. Not like pictures of a skeleton, which can really terrify a sensitive child: if I had had my way as a youngster, none of the infant biology or archaeology books would have been allowed, let alone any books with monsters in them.
if they were being supervised by you in the library, they would only have got the briefest of glimpses, presumably, before heralded off to something more educational? Children often need longer to actually connect to what is going on, so this may well have passed them by.
if they were actually staring at somebody else's computer screen behind their back, you should haul them up about their manners or marshal them away (depending on age); this, like reading over somebody's shoulder, is rude.
if your los were showing an excessive interest, sarky comments from their mum will show what a suitable reaction is
if only pg films are to be allowed, then that will mean that by the time they are doing their A-levels they won't be able to pop into the library to check out productions of Hamlet and Macbeth. By which time you will probably be complaining bitterly about the selfishness of other parents who have taken away this invaluable resource.
My ds is horrified by the thought of wars and of people deliberately hurting each other, and in my opinion this is far more disgusting and corrupting than dancing in a suggestive manner- but I can't forbid these images to be shown around him. All I can do to help him is to say 'come along, we don't have to watch this'.