"One clue as to how can be found in a 2000 study by researchers from the University of Massachusetts. They edited together scenes from the R-rated movies Showgirls and 9½ Weeks that were judged as degrading to women, depicting them as objects to be exploited or manipulated sexually. None contained violence—they included scenes of a striptease, of a blindfolded woman—but they emphasized male dominance as well as female submission and availability. They also portrayed male, but not female, sexual satisfaction.

Half the college-age participants watched those clips, and the other half, the control group, watched a cartoon from an animation festival. Afterward, both groups read a magazine account of either acquaintance or stranger rape. While there was little difference in their response to the stranger scenario, the men who watched the degrading videos were more than twice as likely as the control group to agree with the statement that the victim enjoyed the acquaintance rape and secretly “got what she wanted.” The effect held steady regardless of the men’s attitudes toward gender roles or sexually explicit material.

Orenstein, Peggy. Boys & Sex (p. 62). Harper. Kindle Edition.

So, you're right that it's not only porn (and there is some sex-positive porn out there), but throughout media. But there are probably hundreds of studies out there which demonstrate that easy access to porn from a young age as negatively impacting both boy's and girl's perceptions of what real sex is like, even if they say they know it is entertainment and not real. It also serves to reinforce greater tolerance of sexual harassment, sexism and violence towards women.

"Playing sexually charged video games for as little as twenty-five minutes has been correlated with an increased tendency among college men to say they would engage in “inappropriate sexual advances”—that’s particularly striking, given that, in a laboratory setting, boys would presumably want to portray themselves to researchers as more principled than they might, in fact, be."

Television advertising alone is a 70 billion dollar business because even though we think we aren't being influenced, it's incredibly effective. Porn (and other media) did not become notably more violent towards women because the culture did, although it did boom into a major segment of that industry because it was so popular. But that is kind of my point.

I'm not anti-porn, anti-video games or any of that. I am against the large percentage of thoseand other media that reinforce and normalize the objectification and demeaning of women. Yes, we already have a patriarchal culture that might harbor some of those beliefs already, even if only subconsciously, but reinforcing them does make them notably worse. That's what the research over the past 20 years says.