“Porn” films often do show actual sexual activity, according to every source I’ve ever read regarding the porn indistry. They’re pretty open about that.
However, the point is moot. Society and laws draw no distinction between the viewing of simulated sex and ‘real’ sex, and very little distinction between anything beyond incidental nudity and sex. For example, there is no sex in a Playboy centerfold, or many video presentations of such models. Not all centerfold models participate in movies that display or simulate blatant sexual activity.
The law does not take any note of the ‘reality’ of the act portrayed. It’s all porn, even if it’s just a naked person posing with their own hands well away from their own genitalia.
By contrast, consider the child pronography laws. It is quite a serious offense to possess or view pornographic images (and even text literature, according to many successful convictions - at least one person was prosecuted for possessing self-written stories in their own diary!) Yet it is quite legal for an adult to view images that deliberately give the impression of being underage models. The sites even advertise the resemblance. Indeed, it would be legal (in general) to simulate the portrayal of involuntary underage sex, while the actual act (filmed or not) would be among the most serious of offenses.
“Real sex” (or nudity) is relevant to the laws on pornography featuring underage models, but it is irrelevant to the laws on underage individuals viewing otherwise legal pornography.
*And I don’t even want to get into the issue of underage viewers of pornography of underage models their own age *
Also, appropos other arguments made here, it is quite legal and even popular for PG-13 and -17 films to portray teenagers (real and protrayed) seeming to perform acts that are illegal (even serious crimes) and getting away with it, or even prospering - everything from Risky Business to Porky’s to any number of ‘street gang’ themed films. Even in the PG realm, we expect our children to have the common sense not to emulate everything they see, even if their ‘peers’ are doing it with minor consequences, or none at all.
I think the simple answer, as it is with so many things, is that some parents would be embarrassed (It doesn’t matter if most parents would be embarrassed - they probably would be - but as long as the minority is vocal and can claim ‘traditional’ justification, they will have strong sociological arguments to maintain the status quo.