Why would he be honest with you when you get mad about it and then it causes a fight? Your body image is not on him to resolve. You feeling fat is not caused by his watching porn, so it might be time to understand one does not impact the other. He is obviously attracted to you and gets aroused by you which means you are not fat and gross to him. When you see men in a porn movie with a really big **** does it change your opinion of his ****? Why would it be any different for him?
Another question is why would he enjoy watching porn with you if it causes you to feel insecure about yourself? Your insecurity is far less attractive than anything about your looks. Stop comparing yourself to porn stars because that will help you out a lot. Tell yourself you are desirable and attractive until you believe it.
Believe it or not there are a lot of people who prefer to masturbate and it has nothing to do with the person they are with. Obviously there are some serious issues in your relationship if you're checking up on him and his computer usage. Why is that? You aren't even married yet and you snoop on him more than some spouses after twenty years together, so ask yourself the reasons for that. Has he cheated on you? Have you cheated on him? Do you have your own fetish which you are embarrassed to share and are worried he has something like that? Was this an issue in other relationships?
Him watching porn and masturbating is far less of a problem than if he was sleeping with random people and putting your life at risk. There are ways you can get this to no longer being an issue but it will require you to stop being so negative about yourself. If you continue feeling gross and fat then he will continue to sense the unease and disgust you feel about yourself. There is nothing attractive about insecurity. It is not arousing. You both deserve to have you able to be comfortable in your own skin!
You have to ask yourself... If porn is really the problem. Or if there is problems elsewhere. If you guys still have a good sexual relationship I wouldnt worry too much. Me and my boyfriend have been together for 4 years. He watches porn and so do I. Maybe he is embarrassed about the porn he watches. Fighting isnt going to solve it. Creating a safe space where he can talk to you about it is your best bet. If you feel insecure then thats something you both should work on together.
I'd leave him and find a guy that doesn't watch porn, there's many that don't, you got one that does. You have to decide if that makes you sexually incompatible. To me, it would make him sexually incompatible. I wouldn't take any of what he's dishing out to you. Not much more to say, other than i'm here if you ever need to talk.
Maybe see what he watches and try to do the same thing if he likes rough , anal, milf,(if your comfortable with it) but maybe he's addicted to porn and he just can't let it go, maybe have sex while watching porn.
he likes to watch but you are the one he is with. he may like freaky stuff you wont do. do you like freaky stuff?