It creates performance anxiety.
The unrealistic expectations pornography can create are two-fold, and the women you have sex with aren't the only ones whose self-esteem suffers when being compared with porn stars.
Women watch porn, too. They see men who can go for hours, who are far more acrobatic than you, doing things to women that you can't. These men don't lose their erections. These men finish every time. These men have forearm-length penises. These men are porn stars, and you are just a man. Suddenly, during sex, you can find yourself feeling anxious and inferior. Are you not meeting her expectations?
We're here to put an end to this myth: Women do not want to have sex with porn stars. Women want real sex with real men, even if that means the occasional off night. Take solace in that, and adjust your fears accordingly.