1) Pornography is often accompanied by attack sites with viruses etc. Your machines should be kept up to date and secure, it's true, but why invite attacks?
2) Child pornography. Sure, there may be laws that protect you if it's discovered on a hard drive of a machine you manage, but I'd much rather avoid that mess all together.
Blocking the display of images client side that contain large amounts of uncovered flesh isn't going to do shit to prevent the accidental access of child pornography.
And seriously, how often does that actually happen? This is just another 'think of the children' argument designed to disable critical thinking.
It's not a 'think of the children' argument, it's a 'generally avoid suspicion for crimes with awful punishment' argument. Same as how if there was a known hitman's forum, I'd block access to that too.
Your cite is flawed. Or at least the conclusion you draw from it. The only conclusion that can be drawn is there is a greater number of infected non-smut websites than smut websites. This says nothing about the frequency of infections of smut websites vs. non-smut websites- in other words, it says nothing about the odds of infection given the fact that you are visiting a smut or non-smut website.
1) An image-blocker script isn't going to prevent the viruses from working. It's also certainly not going to keep people from trying -- further, if they can't visit known, safe sites, they'll keep looking and find their way to the dangerous ones.
2) If someone is crazy enough to download child pornography onto a public machine, they're almost surely good enough to bypass whatever protection mechanisms you may have in place.
That's a really good question -- the short answer is it's a really great motivator. Some random pedophile, who wants to obtain child porn, first must learn how, and in the process they would have to learn some basic computer skills, as well as learning to bypass the mechanisms we (rightly) have in place to prevent such material from being distributed, and to catch those who download it. This is why malcontent teenagers who would want to do something like DDoS tumblr are far more likely to know how to do that than well-adjusted teenagers who don't care.
2) Child pornography. Sure, there may be laws that protect you if it's discovered on a hard drive of a machine you manage, but I'd much rather avoid that mess all together.