Porn turns other people into objects. You find yourself thinking about people like they are flavors of icecream. Very few of the people that watch it regularly are as in control of themselves as they think they are. You'll find yourself saying "well, I just like it, I'm not addicted", even though you find it difficult if not impossible to masturbate without it. Even though if you're horny but have no access to porn you decide "nevermind" versus using your imagination. Even though when you've tried to not watch it, unless something physically prevents you from being able to, you eventually give in because you can't take it anymore. Even though the level of arousal you get from fast clicking through a few dozen videos is far beyond what you're able to achieve with your partner so your brain starts inventing reasons to explain that. Porn has been harmful in the life of every person I've been close enough to see the effects of it with. But it is also only outshined by drugs as far as the pleasure response it gives your brain, so it is very difficult for most people to admit it causes them any problems because that plants the seed for giving it up, and those dopamine spikes are hard thing to want to be without.