Warning: all italicized material following is my reconstruction of the stance. It's not my stance, but it's not their exact words either.
Kids too young to read, and kids young enough to obey their parents' injunctions to stay away from fic they're not old enough to read, goddamn it!, and kids mature enough to seek it out and enjoy it are not the ones we want to lock this away from using passwords and some form of secure age authentication. Kids who are too young for it, it'll pass right over their heads. Kids who are mature enough for it will appreciate it and thank gods for it being there. It's the gap in between those maturity levels that concern us.
The age/interest group that we want to protect most is those kids who are barging around on the internet deliberately seeking out R and NC-17 content, not because they actually can use it, but because they just want to stir up trouble. They'll read the warnings, and they'll go ahead and read it anyway, because they know they're not supposed to. They're the kids who break rules just because they're there to break, not because they care what the rules stand for or what the rules are there to protect them from.
They're at an age and experience level where they may understand the idea of sex, but they don't have context for it yet. They can't yet sort out "This is pretty typical" from "This is some seriously fucked-up shit" because while they may have had the theory, this is as close to the practice as they've ever been, and there is no one there telling them what's right and what's wrong. "They could go to their parents and ask about it," yeah right. Have you seen some of those parents out there?
And, more to the point, those kids. Those are the kids who would never dream of going to their parents if they ran into something disturbing. They'll process it by themselves, and probably do a fucked-up job of it. Like it or not, if your sick incest story is the first thing that kid runs into that's not school textbook biology, that's what they're going to have as their first impression of explicit sex. It's not the kids with the good parents that we have to worry about. The good parents are watching the kids, and making sure they're well-educated enough to handle whatever they run into, be it bad or good. You don't have to worry so much about the parents who are watching their kids like hawks. It's the parents who just don't care what their kids are doing, the ones who let their kids do whatever online, that have sufficiently fucked-up relationships with their kids so the kids can't trust them to talk about things with. Those kids are the ones who need good education the most, and those are the ones least likely to get it. The only hope for them is to keep the weird stuff well enough out of their way that the odds are that they'll run into an educational site when they're looking for explicit materials just because they can get away with it.
It would be fine if everybody was educated in how to handle sexually explicit materials, but the fact is, they're not. And I really don't see that happening any time soon, with the fucked-up families that aren't communicating with each other, leaving schools to do all the teaching. It sucks shit that fanfic authors have to do the work that parents should be doing, but the fact remains that while the parents aren't doing their job, someone has to be.
Something more to think about. The only reason that this is not my stance is because this is not my experience; my experience is of being one of the kids who was mature enough to handle it (and need it), but not comfortable enough with the parents to ask specifically whether reading adult material was OK.