The main disadvantage to watching pornography is usually considered to be the fact that it gives people unrealistic expectations regarding sex. It must be remembered that the people in porn films are actors and they are doing a job, that is not necessarily how they will have sex with their partner. The sex in pornography is a show that is directed in such a way that it entertains and stimulates the audience and may not necessarily be something that the actors are enjoying.
Pornography and body perceptions
The actors in porn films often have bodies that we consider to be 'perfect'. The girls tend to be pretty with slim bodies and large breasts and the men tend to be muscular and good looking with large genitals. This is the reason that they are porn stars and is not a representative of how most people look. This can make people sensitive about their looks and make them believe that they are meant to look like this.
Pornography encourages promiscuity
Promiscuity is not always a bad thing but it is often argued that pornography makes it appear that having sex with multiple partners is a good thing and some people do not agree with this and think it irresponsibly promotes multiple sexual partners and possibly unsafe sex.
Advantages of watching pornography
There are some advantages to watching pornography as it gives people the opportunity to explore their sexual side in a safe environment. Some couples may wish to watch pornography together to put a bit of spice into their love life and encourage one another to be more experimental in their love making.
The disadvantages of watching porn for some people is that when they actually have sex, they are not totally sexually into it as they use to be due to being so use to seeing it play out. It's not a turn on anymore. I believe the human body needs some surprising excitement sometimes in order to enjoy sex.