Here are the reasons
1 – IT IS DANGEROUS TO THE WATCHER.
The damaging effects of watching porn are well documented – with watching porn being linked to cocaine or heroin in terms of its addictive effects. The more porn that you watch, the ‘harder’ porn is needed to get the same ‘fix’ in the future.
2 – IT SEXUALISES CHILDREN AND PUTS THEM AT RISK
Pornography is also associated with early sexualisation of children. The average age of first exposure to pornography for boys is 11, at which age their viewing may include violent porn, bestiality and child pornography. What has been seen cannot be unseen. The alarming increase in child-on-child sexual assault is directly correlated to increased exposure of children to pornography.
3 – IT EROTICISES VIOLENCE TOWARDS WOMEN
A 2007 analysis of hard-core pornography revealed physical aggression in over 88% of the scenes, but women showed negative responses to the aggression in only 5% of scenes. 
4 – IT IS DANGEROUS TO THE ACTORS
Women whose prostitution is filmed for pornography often suffer more severe symptoms of PTSD. Their exploitation haunts them for the rest of their lives. It continues on film, played and re-played over the Internet indefinitely. Once the actors are in the industry they have higher rates of substance abuse, typically alcohol and cocaine, depression, borderline personality disorder… They have to be drunk, high or dissociated in order to go to work. 
5 – IT IS SOCIALLY DAMAGING, PARTICULARLY TO WOMEN
Pornography teaches young women and men that women are valuable only to the degree that they can satisfy and submit to the lustful, objectifying, even hostile, sexual dominance of men.