It can create unrealistic expectations about women’s bodies.
Most of mainstream porn fetishizes different parts of the human body, making for an exaggerated "exquisite corpse"-style composite of a woman. The typical female porn star is built like a Barbie—hairless, with massive breasts, long legs, and a heart-shaped butt. Also, bleached body parts.
Confuse fantasy with reality, and you might find yourself disappointed when the women you encounter on a day-to-day basis fail to live up to the hyperbolic sex of the adult entertainment industry.