A liar thinks everyone is lying. A porner thinks everyone does porn. Not true.
Anyone with the vaguest realistic understanding of the actual human condition since, well, the evolution of humans (male humans in particular) isn’t somehow shocked the figure is as high as 70% for 18-24 year old males; they recognize almost all of the remaining 30% lied to the survey takers out of embarrassment. For anyone left, they probably have some sort of medical or mental condition.
We live in a society where for a variety of reasons getting married before 24 is generally frowned upon. Even if you don’t agree that it’s unrealistic to expect 18-24 year old men to abstain from sex; it’s silly to the point of absurdity to expect them not to look at any porn at all.
You’re trying to apply a set of sexual moral standards from when people were getting married at 15 or 16; we’re never, ever going to go back to that.
And actually what porn is probably replacing is prostitution; back in the “good old moral days” that was rampant, despite whatever you might imagine.