It's impossible to weigh the pros and cons of most things, porn included. I could easily think of 10 different ways it could affect someone's sex life either negatively or positively. To answer your other two questions, you can have a compulsive behaviour that negatively affects your life, which is what most people would call an addiction. Search wikipedia for Psychological_addiction#Behavioral_addiction to get some more details.
By: Bunner
+202reps
Very sensual for an unmarried girl. It made me turned o.