They are right in that teens will always be looking for porn, even just out of curiosity. They'll find ways to access it, even if heavily regulated and secured. Working at a school, I can tell you I've seen some pretty explicit stuff on some kids' devices. No matter what you do, you can't prevent it, short of completely disabling access to electronics, and posting security at every venue that makes these things available.
That doesn't mean one should be making it publicly available. Not everyone is ready for that kind of stuff.
And it's a particularly vile argument if they are talking about pedophiliac content (depicting underage people).
But they are also not wrong when they say that it's better to let kids experience this kind of content in a safe environment. Ideally, that should fall under parenting though. I think that when kids go about it in secrecy, they end up harming themselves emotionally and mentally.