I wrote a kind of long description of the aftermath of this. I'm going to copy and paste it for your input. Thank you.

Is it a bad sign that it caused a problem? I asked, he was fine with it, I said cool and felt good that it wasn't a big deal for him. Fifteen seconds later, he got defensive. Like, why do you need to see it? I don't understand your motive. Why do you specifically want to see my messages with her? It doesn't make sense to me. I'm very uncomfortable with you wanting to see these messages. And I was trying to explain that I think she is flirting with you and has a crush on you and maybe you aren't aware of it. Like, maybe I can weigh in on this and we can work on boundaries if that's the case. But it kind of became an argument. I did read the messages. Didn't really like what I read. Their friendship went way deeper than I was led to believe by him, and there were some places he had gone in the past with friends, except I was in the dark that she was one of the friends who was present. Even looked like he might have gone to her house before, though I couldn't tell. In his words, he never gave me total details to avoid drama that I would cause because I was already suspicious of her intentions. I'm conflicted now. Things are weird between us. When I asked him to explain the messages, he refused to answer because "I wouldn't believe him anyway." Eventually, he did. But the whole thing makes me feel awful. It went from me questioning her intentions to questioning where my boyfriend's heart even is. He had perfectly good explanations for the occasions. Like, I told you I went to a dinner party that day. I just didn't give as many details. I didn't think you would care to know she was one of the few people I went drinking with that night. Etc. He said I had trust issues. I don't know what to believe. I want to believe this is all innocent, but my gut is telling me something is going on here. Sorry for writing so much. I am very hurt right now. I never thought he would be capable of cheating on me emotionally (if he is) or hiding things. I've gone two years thinking we had this super solid and wonderful relationship. I don't really know how to proceed. In his words though, if he was trying to hide this stuff, he could have just deleted it. He agrees he should have given me more details about their friendship, but he didn't want my reaction to cause friction between us. I'm torn. I don't know what to do.