I've had extensive chats about this with my boys and the claim that it is completely normal for preteens seems overblown to me. My youngest is 13 and not yet all that interested in girls. (His friendship group are very young compared to others.)
My middle one however was worrying me at the exact same age, to the point where I insisted on meeting the mother of his girlfriend so we could agree some ground rules. I do know he still didn't watch porn at that age, and didn't need or want it either.
An awful lot of talk at this age and throughout the early and midteens is bravado as well. And there is a huge difference between "they've seen porn" and "they watch porn".
I know many kids this age will now have been shown porn or come across it googling. DS3 and me were searching for some children's cartoon when he was about eight or nine, for instance, and what came up was porn created by some sick and twisted bastards using the cartoon characters. He didn't even understand why I was clicking away so fast, but it was obvious to me what it was. (This was despite parental filters.)
That doesn't mean the majority of boys that young watch it regularly. Or that they all know what these extreme practices are unless they are indeed being taught them in school. That's a sure fire way to sexualise children.
However, porn is increasingly an issue because a) it has changed enormously in the last 20 years and many parents remain unaware of this fact. They often think "housewife and plumber" porn and don't get our outrage, but what is on offer now is far far worse.
Hurting, humiliating, denigrating and abusing women was once a fringe interest in easily accessible porn. It is now mainstream, to the point of 95% of porn on the main sites falling into that category.
And b) because of this, it distorts too many older teenage boys' emerging understanding of what real sex is - it isn't a vehicle to have your needs serviced by submissive women who obey your every command or get abused otherwise (without consequence) but a mutually enjoyable activity both partners freely and happily consent to and which stops when either of them doesn't. And which might not happen at all because real women aren't always in the mood, especially if you've behaved like an idiot.
So no, in my opinion it's not normal for younger kids to watch and I remain suspicious of the motives of anyone who claims it is, but it is certainly an issue among older teens. One that we as parents must address in my view.