While pornography itself isn't really a bad thing, it's a pretty unrealistic sex. It's glorified and fake and only focuses on the physical side of it, not the emotional. Teach kids that pornography is unrealistic, and the risks of stds and to always use condems during actual sex. Whoever thinks this is a good idea must be a complete dumbass.
The sex you view in porn is so over-the-top it's almost laughable. I would say it's as simple as talking to your partner about what he/she likes. No one person is going to be the same as far as sexual preference goes. If you try to reenact porn sex in real life, there will probably be problems.
In most films, it portrays women as sex-obsessed toys for men (or women) to play with as they please. This will seem right to a child who has had no experience/knowledge on the subject. They may even reenact what they saw, depending on what it was. It's the opposite of good. It's harmful and potentially dangerous.