If you fill your mind with pornography, it's going to negatively influence the way you think about female and how you treat them. Pornographic memories will never influence you in the right direction when you're in a relationship. Sexual desire is a fire that must be kept under control. Pornography pours fuel on that fire.
When as a male you lost after the female you sees in pornography, the images burn into your memory so deeply that its never totally disappear. And lusting for women is not just a mental thing. Jesus said in Matthew 5:28 that ...whosever looks on a woman to lust after her hath committed adultery with her already in his heart.