It can desensitize you, emotionally and physically.
The trouble with watching porn can occur when it becomes an inextricable part of your masturbation routine. Routines are hard to break.
You touch yourself a certain way, while watching a certain thing. Suddenly, when a woman comes into the picture, you may find it takes longer to orgasm, if you even orgasm at all. Without those routines, and specific cues, your pleasure can feel notably dulled. The issue might be that you've created an experience with porn that is difficult to replicate.
Even if you're not consciously looking to re-enact what you watch, your body can come to expect certain things, certain ways, making it more difficult to aroused when you're doing things differently. You know, like without your computer, and with another person.