Why does my boyfriend watch porn while I am home/in the next room?

I have been with my boyfriend for about 8 months now. I am 21 and he is 25. We have gotten in several fights about his porn. i have caught him in the act twice and have seen him look at dirty galleries on ebaumsworld or break.com multiple times. looking at the dirty albums usually happens when i am in the same room but cant see his computer screen or when i am in the room next to him or outside smoking. i dont understand his logic behind looking at this when i am litterally right there. we have a good sex life and outside of this fight (which we have had 3 times now) we have never argued and are constantly happy together. i have confronted him about it and he told me he wouldnt do it because it makes me extremely uncomfortable but today i caught him. he clams up when i try to talk to him about it. i am at the point where i just want to move out or even end the relationship because it makes me extremely insecure. i have communicated all of these points to him and he promises change but i have only been let down. every morning when i wake up and he isnt in the bed i feel like i need to make noise or shuffle around so he knwos i am up and if he is doing anything dirty he has time to quit it. but when i step out to smoke a cig and in the 5 minutes i was gone he goes and looks at dirty pictures i feel at a complete loss. please help.