I think because in many cases porn can be exploitative and can perpetuate stereotypes of aggression in sex and set unrealistic expectations for many people who then develop complexes.

It's a complex issue. On the one hand sex is natural, on the other hand the way it's portrayed can have major influence on how young adults (and even mature adults) view sex which can be loving and consensual to violent and unrealistic. And it can lead some to objectifying people.