OP, I completely understand. My boyfriend is a lot like yours. I don't have anything against guys watching porn - I'd actually prefer that my boyfriend watch porn than not, because that's usually normal male behavior.
But, like you, I also started getting a weird negative feeling about it every time I found porn on my boyfriend's computer, tissues in the wastebasket, that kind of thing. I was in the living room on my computer last night, and he must have been watching porn in our room while I was out there. What got me about it this morning (I had to use his computer to scan some documents and email them to myself) was that I was right there, and he'd probably locked me out while he was jacking off. I was hurt at the idea that he was doing it behind my back, even though he's never denied that he does it and has always found me desirable.
Your problem sounds a lot like mine. Maybe it's just how women are - emotional about sex, rather than purely physical. Neurologically wired to want to keep our men all to ourselves so they'll stick around and help us raise the babies. I don't know if that's really what's going on, but it helps to keep telling myself that. I also try to focus on the fact that I'm glad my boyfriend has a normal sex drive, and remind myself that sometimes I also want to get off without going to the trouble of having sex - and that when that happens, I just do it. So whatever negative feelings I have are probably just my inner cavewoman trying to monopolize my bf's sperm, or something equally primitive/hilarious. My baby clock has started ticking, after all. Better to laugh at it than cry about it, right?
At the end of the day, I think the best way to handle it is to just keep telling yourself something that feels okay to believe. Because one way or another, men are going to be men and they are going to watch and want to watch porn, whether we women are included or not. And if our relationships aren't in trouble, why create problems? It's not worth the drama. Best of luck!