AIO for storming out of my boyfriend’s truck after going through his Snapchat messages?

So for context, I've made it explicitly clear to him that one of my boundaries is that neither of us should add random people of the opposite gender on our socials. This excludes friends, general acquaintances, colleagues, family, etc. We've been together for 6 months, and he's well aware of how I feel about this. He has taken some liberty to unfollow women he doesn't know or talk to on Instagram. I don't follow or accept any men on any of my socials either, and I didn't even do this before I met him because I'm simply not interested. We were watching a video on his phone today when I noticed him immediately swipe away a notification, so I asked him what it was. He told me it was nothing and continued watching the video, but I got curious and swiped down to see his notifications. I saw that some random woman he didn't know was posting on her private story, which he had been added to. I asked him who she was, and he claimed he didn't know her. I then asked him why he would even keep her there if there was no reason to. He just unadded her and said that he added lots of people a long time ago. This made me upset, so he offered to let me go through his phone, which I did. While going through his Snapchat, I found conversations with multiple women. There was one conversation dated about 5 weeks ago that I opened, and I saw that he had been talking to this girl romantically before we started dating. What really caught my eye, though, was that one day before we made things official, he was asking her what she was looking for in a man, along with other romantic things. After we started dating, he was still seldomly talking to this girl, but he claims it "wasn't romantic." From the messages, she was once obviously trying to flirt with him and my boyfriend swears that he didn’t know it was flirtatious. (She asked if her hair looked better up or down, and he said down.) Seeing all of this made me explode. He said this was "purely conversational" and that he didn't have any romantic intentions behind it. The most recent messages from five weeks ago were along the lines of, "Hey, how have you been? How's the job going?" etc., but there was no mention of him having a girlfriend or anything. I already felt sick looking at all of this, so that's when I stormed off. He called me nonstop afterward and practically just said that he would never cheat on me. He thinks his actions weren't inherently cheating or intentionally hurtful, but he understands that they hurt me and said that he'll block any girl he isn't in close contact with. I forgave him, but I still feel really shaken up by all of this. What do you all think?