I am anti-porn due to over use. So that is my inherent bias but i will give non-overuse issues.
Point number 1: Perception of sex.
Porn has become the way we learn about sex since generally the population of the U.S. at least looks at porn before becoming sexually active. As such porn influences how we have sex. The problem with this is porn is generally emotionally and physically abusive.
The problem with this is we think our sex lives are awkward sexual poses and emotional and physical abuse. On top of this males who use porn are trained to gain sexual pleasure without being trained to value of a woman attaining pleasure. So this makes your brain through "training" able to get off sexually without any significant pleasure of your partner. This leads to an unfulfilled sexual relationship as it is only about you getting off and not about sharing the experience with your significant other. As if that wasn't bad enough it also changes your perception of gaining sex with women within our culture. Women see relationships as a gradual progression toward affection, but porn teaches us we can rush right to sexual relationships without issue and this actually influences our dating habits! You can ask women on this forum about that statement trust it is true! This change in behavior actually hurts our chances of successful consistent mating or just mating in general.
So I talked about why the viewing of porn at an early age is bad because it defines how we have sex, and our sexual habits with our partners. Physical and emotional abuse should not be part of a healthy sexual behavior, but can be mistaken for the norm because within our society true intimacy is seen as obscene while porn themes are shown as normal. For those of us who are bad with women this adds another layer of against us by giving us the impression that rushing into a sexual relationship is normal when it really isn't. So these are just some of the reasons i think porn are bad.