Greets!
Gender and sexuality has always been a issue to identify properly, especially with all the assumptions being made on what is good or bad in concept. Perceptions of experience also have an effect on assumptions made and identifications reached on issues of both sex and relationships, as well as the existence of experience at all.
For some, the latent connection to other people may be so autonomous that they simply cannot stop connecting to those within their community and beyond, resulting in relationships relating to intimacy to flourish with or without their input. Similarly, someone's autonomy can draw them away from others, making intimate relationships be quite literally something of legend to them.
Porn has been something that has been around for a very, very long time and if an individual's experience of a sexual relationship is porn, then they may find that they'll automatically link porn and relationship together. While it can be said that porn is fiction, people that have been recorded fucking for your entertainment may very well be real, even if they arn't really enjoying it. This realness may actually be what a watcher may be attracted to, even to the point of imagining themselves having something they may not have.
This is where the unrealistic expectations may come from, not that the parties included in the dirty simply cannot last anywhere near as long as mister longah peenus mc orgasmatron, but that those with these expectations have contructed something unrealistic in what happens before and after the deed. This isn't only limited to lonely 'nice guys who get no action' people either, expectations of a relationship stretches beyond gender, experience and orientation.
Ultimately, I believe that porn can never influence expectations in sexual activity, because when one goes at it, the activity itself is not what is being focused on, it is the other person/s. Even if the partaking individuals are not fully aware of it. These things happen for a reason and unrealistic expections are constructed for a reason, an important reason which those who suffer these expectations may be able to answer, if they search hard enough.
Unfortunately, I personally do not see many people who suffer these expectations to actually try to answer; Why?