Kids can be remarkably skilled in bypassing 'Net nannies. At our local school (elementary) I used to run an after-hours class in one of the classrooms. One day the kids were in the computer room supposed to be working, but when I wandered in there they were showing one another various pics and clips they'd downloaded. Most of these were very inappropriate, but had been accessed on the school's server. I quietly observed, commented that it wasn't what they were supposed to be working on ("so please come back into the room now and we will get on with things") and next school day, I dobbed in this practice and warned the school to tighten up.
It can be easy to stumble onto the wrong stuff. Some years ago I was researching a character in Greek mythology, Daedalus, when I got a hit very high on the list. I clicked on it and it was a site for foot fetishists! The site was managed responsibly, it had warnings to not click any further if you were easily offended. I actually did (out of perverse curiosity; my chosen research topic had just been mysteriously hijacked, after all) and didn't actually find anything offensive, although the concept of some of it was a bit disturbing.
When I found difficult child 1 viewing porn, I educated him about the downside of the porn industry. We started by Googling "dead porn stars" and went through some of the ways in which people have died. A lot of them died of AIDS. Others died violently or of drug overdoses. The porn industry especially chews up women and spits them out. While there are going to be some who get in to the business, make money then get out with their health and sanity intact, these appear to be the minority.
I then emphasised to difficult child 1, that viewing the stuff is encouraging this industry that damages people. To view it is to become part of it and therefore to endorse it and everything that they do.
Kids in their teens often try to up the ante with regard to shock value. They do it so they can tell their friends about it next day ("guess what I saw online last night!" and bignote themselves in terms of what they have experienced or can handle. You can picture it - kids talking in the playground, one says, "I saw a clip online last night of a man and a woman having sex! You could see EVERYTHING!" and the next kid says, "Oh, that's so ordinary. The 'Net is full of stuff like that. Try Googling 'naked gay sex'!"
I have a BFF who was very innocent during her marriage, she then led a celibate life for another ten years after her husband left her. She had NO clue. Then I mentioned one day that we'd had a very nasty catalogue stuffed into our letterbox, husband was actually about to take it to the police because it was so rough, and it's illegal here to send this sort of strong sexual material by snail-mail, unsolicited. BFF asked to see it first, she was curious. I warned her, "It IS strong stuff."
She saw it with a mixture of horrified fascination and disbelief - "people actually BUY this stuff? People actually DO this stuff?" She didn't even think it was anatomically possible, in some cases.
The other horrifying thing that can so easily happen with kids surfing the 'Net unsupervised (and if they want to do it, they will find a way) is when they can put their name down somewhere or leave a contact somewhere that means your email address suddenly becomes bombarded with spam, especially spam of a nasty sort. husband is "out there" internet-wise, and so one of his email addresses does cop a lot of nasty spam, especially the ones offering women who want to meet a man. He showed me one such email which was basically saying, "Hi, big boy. I enjoyed our time together when you visited my country. Remember when you told me to look you up if I was ever planning on coming for a visit? Well, I've decided I'm on my way and I would love to arrange to meet up with you for some more of what we did that night..." kind of message. Only a lot more graphic, promising transports of delight of the carnal kind.
It is purely a phishing expedition, but a jealous wife would get a lot of exercise jumping to the wrong conclusion.
I hate to think how difficult child 3, for example, would react if he opened one of these messages.
Marg