Older boyfriend breaking boundaries… right?

My boyfriend is 51 and I am 24 (F). We’ve been together for close to three years now. During that time we’ve experimented with being “open”, aka going to swingers clubs and welcoming other singles and couples into bed with us. I’ve really enjoyed it so far, and we’ve become very close with three other couples who we party with and occasionally swap with. Again, with these people who are my friends, I absolutely love it.

A while ago I found out he was talking to someone on tumblr and they were sending him nude photos. I got so upset at this because they were a stranger to the both of us and it felt like it was close to cheating because I had no idea it was going on. He apologized for it and said that he didn’t know it was a boundary of mine, he didn’t think it would be a problem, etc. Whatever. Maybe he’s being truthful because of our sexually open-ish relationship. I made my boundary clear at that moment and thought we moved on.

Fast forward to about a month ago, he finds a couple online and wants to set up a play date for all of us. At this moment I was feeling kind of insecure about myself due to some unrelated things and told him I wasn’t ready. He said ok, but he had already added the couple on Snapchat which was fine with me. But I specifically said to him, please no photos exchanged. He said of course not.

Two days ago I’m using his phone to give him gps directions while we’re driving and he gets a snap from the girl in the couple. Surprise surprise it’s a topless photo. I lost my shit… I feel so broken that he’d do this. I asked him why. I asked him if I wasn’t clear enough when I asked no nude photos. He said no, you were clear, I was just being stupid and wasn’t filtering my messages well enough, and he felt stupid that he let those ones slide.

I feel like he’s playing dumb right now. How could he not know that this would upset me? Am I missing something here? Am I expecting too much monogamous behavior from a non monogamous man?