Trust

Hi. So basically I've been with my SO for nearly 4 years. We're pretty open with each other but earlier this summer I found porn on his phone. I was so shocked and hurt because he really shows no "over" interest in things like that. Like he's probably only kissed about 4 girls because he's very careful of that etc where I've been with many guys. Anyways he was very embarassed and upset when I confronted him about it. So after a few months I just try to bring it up casually as I am ok with it , I ur wish he discussed things more! What's made me post this is that accidentally I left my fb logged in and he read some message about me saying how some guys and men were attractive and he was really upset by this (but didn't tell me until a drunken row). As I said earlier he was so upset because he said he would never comment on other girls and I was on other guys. Basically what I'm getting at is that I'm trying to say to him that we need to be more honest about this kind of thing, and how would I go about doing that without an argument starting? I don't want him to be so shy over discussing sex with me as I find it worse that he "hides" it. I also feel terrible that he saw that and it hurt it so much, but I don't think it was out of line? surely I can say when someone is good looking. I would never cheat on him, I love him to bits but he seems to take little things like that to heart