Why do some women get very angry if their husband watches ?

Ive been married for 2 years and Im 22. My husband is 32 and he watches every once in a while if Im not home or if Im asleep and hes bored. I watch it way more than he does. My girl friends always complain that their husbands or boyfriends have on their computer and one of them said that she put a block on the computer so he couldnt look up porn and that if he did she was going to throw all the computers out of the house. I dont understand.... Why would any women be mad if her husband watches porn? Everyone has watched it before. I cant fathom why anyone would be upset with this unless he had a problem with it and enjoyed porn more than . Whats the problem with porn?