Quote from NishTo say that pornography is harmful because it doesn't portray sex realistically is like saying that Norman Rockwell paintings or sitcoms from the 50s are harmful because they provide an overtly chirpy, sweet, happy view of family life which is wildly inaccurate.
I'll say right now I'm not trying to claim that pornography is immoral or that it is always going to be harmful, but there are times and places where it can be harmful.
Pornography addiction can be very much harmful.
Going back to what I said about expectations:
"What are some of the problems that people can have with using pornography? The biggest problem that people often have with using pornography is that they sometimes start to expect their own actual sex lives to be just like the pornography they use and enjoy. This is really pretty ridiculous and unreasonable! Pornography is idealistic, not realistic. Porn tends to show what people fantasize about, not what actually does happen in most people's sex lives."
"When people start comparing themselves or their sex lives to what they see or read about in pornography, they can often start to feel inadequate or maybe even abnormal. Sometimes people try to compare their partners' bodies, sexual desire, or sexual responses to what they've seen or read about in porn. Both are really unfair and useless. Pornography isn't what real sex or real people are 'supposed to be like,' it's an idealized version of someone's sexual fantasies. It's fine to use pornography if you want to, but remember that porn is not real life and you and your partner(s) don't have to 'measure up' to it!"
http://www.scarleteen.com/article/gender/looking_lusting_and_learning_a_straightforward_look_at_pornography
I guess pornography is harmful in the sense that a bunsen burner is harmful, if you haven't been taught about it before hand you can end up very badly affected. The key difference between the two is that you pretty much always get taught not to put your hand into a flame, where as there is such minimal discussion about porn the same learning can often not happen - and that's when we get problems.