Boys will be boys! You can't expect guys not to look at, flirt or talk to other guys. It's unreasonable, and it'll never happen, no matter how in love you two think you are with each other.
I actually had this happen to me during my last relationship. My ex was/is a huge online-porn addict. He would spends four to six hours a day online looking at bodybuilding porn, bodybuilding websites and be chatting with his online friends from Facebook and bodybuilding websites, etc. At first I tried to "love" him unconditionally and not judge his addiction, but at the same time help him and not feel depressed because he was more into that lifestyle than our relationship.
After a while the curiosity got the better of me. He left his Facebook page open on his laptop computer, and I went to use it and saw that he had left his page up. Yes, I should have been a good boy and closed it, but I, at that time, was severely depressed at where our relationship was heading and lost all trust anyway, so I looked. He was engaging in sex talk with many of his Facebook friends, scheduling future meet-ups, and even sending nude pictures of himself to guys online. I was hurt beyond belief and extremely angry.
I confronted him about it, and of course, he was upset that I had invaded his privacy, and rightly so because I should have not did that, but it was too late to undo what I had just did. He explanation was that he never cheated or planned on cheating. It was "innocent" flirting and that I shouldn't get all upset because he was with me and didn't want to be with anyone else. You know the typical BS you get whenever you catch a partner cheating.
Long story short, the relationship ended shortly after that because there were too many issues between the two of us. It made things even worse now that the trust eroded between the two of us. I didn't trust him not to cheat or to stop his online cheating and flirting, and he didn't trust that I was not going to keep track of his cheating.
Looking back at it, I was insecure with our relationship because of issues that I won't go into, but the online cheating made the issues far worse. I do agree with my ex that he wasn't actually planning on cheating or meeting these guys. He is into bodybuilding and has major insecurities about his looks and himself, and he is always looking for validation from an outside source. For him, having guys flirt with him online gives him a much needed ego boost. He feeds off of these compliments from other guys, and there was nothing that I could do or say to make him feel better about himself because he needed constant attention from others (yes, he is a narcissistic sociopath). He wanted, or needed, to hear all of these compliments from complete strangers in order to feel good about himself. I could tell him every minute how big he is and how hot he is, but it wouldn't matter coming from me.
In my situation, my ex's cheating and online-porn addiction problems revealed his extremely low self-esteem and the issues that he's had with guys his whole life. It was NOT a poor reflection on me or what I wasn't doing at home or in the bedroom. He had these issues way before I met him and no matter how much love, support and empathy I gave him, nothing I did could help.
Don't ever blame yourself for him looking other places, especially if you're there for him because it's him, not you. You have to make the decision whether to stick by his side and work out the problems or just walk away from it now, and it might be the latter because it seems the trust has already left, so there's not too much left to work on.