Unexpected P

So.... My SO has been doing amazing the past couple months. It's been since May that he PMOd or viewed any videos or explicit photos to the best of my knowledge. It's been a good few months without Psubs. There is still M going on, but our sex life isn't suffering because of it and our relationship is st it strongest right now so I haven't been too worried about it.
Now, FB messenger. The toxicity. Having single male friends who act like 13 year old boys my SO receives gifs and photos, sometimes videos from them.
Before we were in a good place he would hide this from me, he'd gaslight me. It was hell.
Today he got a message for the first time in a while. I got home from work, set my things down. He greeted me, hugged me, all is good and he just said "I just need to tell you I just got a message today, I didn't know what it was so I opened it, but I deleted it right away". Which is true. EA shows messenger was opened literally 1 minute before I walked in the door and the conversation history was deleted as he showed me. I told him to block him on messenger which he didn't protest for a second and we did so.
However this is a huge trigger for me. Especially because I don't know what exactly the message was. It makes me feel unsafe and turns me off of affection. I know this is something I need to work on and have been. And I've come a really long way.
He handled the entire situation amazingly however. I am actually shocked. He is a completely different person than he was before.
I just told him thank you for being honest, I love you, I just need a few minutes alone to process.
Does anyone else get P by accident like this? I know he isn't asking his friends to send it, and he has never replied to any of the messages indicating to them he enjoys it. We're both doing the work to have a healthy happy relationship. But what do you do when the outside world intrudes with P out of nowhere? A long time ago I asked him if 20 years ago his friends would snail mail him P or come by to drop a random picture off. He laughed and said no because obviously that's absurd. But really doing it online is no different. Just faster.

 

He hasn't told any of his friends or family. And I can understand that. It's embarrassing and men are "supposed to" looking at that stuff 24/7. I try not to let it get to me but it's irritating and really sets me back. Like I don't want him to touch me today because of what he saw, even though it was just for a minute.