As a disclaimer, the points I have to make on this topic have no basis on actual research (that I'm aware of); it's entirely based on theory and anecdotal accounts I've heard from people. But I think some of the most detrimental effects of porn (particularly mainstream porn) are the ways it teaches young people (i.e. people who don't have other models for sexuality) that "this is the way sex works." If we all talked about sex openly and honestly, and came to understand the vast diversity in people's sexuality, then I don't think porn would have the same effect. But we're all so quiet about sex, especially with adolescents; we don't teach them about the good parts, the concerns, the ways it interacts with your self-worth, and just how different people are in terms of their sexuality. We just teach them "this is how you have safe sex" (if we even teach them that). So given that we don't all go around seeing each other naked and having sex, we don't really know what's normal. For an adolescent boy whose only interaction with sexuality is mainstream porn, especially one who doesn't have a lot of female friends, you can see how ideas of sexuality could get warped. Without a lot of real life experience, boys can develop this mindset that "this is how men act during sex," and model that during their sexual encounters. They can also learn that "this is what these body parts look like," and can be surprised or even turned off when they see something that deviates from that.
You wouldn't believe the amount of insecurity I've seen in women around how their labia look. I don't think that would be possible if porn hadn't created a standard for what labia "should" look like. It's definitely not restricted to labia, either; porn furthers the notion that large breasts and large penises are socially desirable, and that can create a lot of insecurity. The amount of communication online makes it easier to learn "the real facts," but it's still easy to look at porn and feel inadequate for both men and women. As we grow up and become more mature, we discover that the standards mainstream porn creates are unrealistic, but that insecurity can still linger.
Outside of issues around understanding how people's bodies are "supposed to" look, I think mainstream porn creates sexual scripts that are counterproductive to good sex. Again, this is primarily a problem among people who have no other models for what sexual intimacy looks like. But by having the men be hyperaggressive and demeaning towards women, and having the women act like they're enjoying it, porn can create an expectation that that's typically what women enjoy. For boys who empathize well with women and are generally communicative in their relationships, this won't be a problem; for boys who don't, it can be a huge issue, particularly if the woman feels insecure about speaking up about it (and gender norms that encourage women to "keep the peace" don't help with that). Among things that mainstream porn paints as normative are "the jackhammer" approach to sex, men basically expecting that women always want to give oral sex, and teaching men how to give the worst oral sex possible to women. Also, no foreplay or communication...ever.
This isn't inherent in pornography; I think there's a lot of great porn out there, and I believe porn could be really useful in helping people see the diversity of approaches people have to sex, and figuring out what really works for them. But the majority of porn doesn't show that; it shows an approach to sexuality that most women don't find pleasurable, and it fosters insecurities about body types and sexual performance (even more so than the insecurities that are already there when you're new to it!). For a lot of people who watch porn (possibly even the majority, or the vast majority), it's not an issue; they learn how to communicate with their partners and, through experience, develop realistic expectations for their own and other people's bodies. But for some people, it encourages a highly destructive sexual dynamic and creates unrealistic expectations for everyone involved.
Porn addiction is a real problem too, because people who get addicted often tend to "escalate" (they need more and more intensive and extreme porn to be satisfied). It's a related problem in some ways, because it's easy to think that being into really extreme sexual things is "just who you are" once you hit that stage, especially if you've never had a stage in your life when you didn't watch porn to discover that you can be totally satisfied with a relatively vanilla sex life, and that expanding to "less vanilla" things can happen gradually and as you and your partner(s) feel comfortable.