I'd guess that PP is correct that more porn makes it to computer screens at the GPL than is documented by incident reports. I would expect the usage numbers to support this supposition when we see them this fall.
I'd also guess that people fall asleep in the library and get in fights at the library and whack off in the stacks without getting caught or reported, too.
If you think that any instances of anyone ever viewing porn on a library computer justify any method of response, however flawed it might be, then you're going to end up voting to block email and Facebook.
Short of that, you're looking for some balance that keeps libraries safe and welcoming. Or you're playing politics.