@July26 I think you need to separate out two issues here, because they’re getting tangled together.
The videos themselves. They’re years old, long before your DP, and lots of people have old files buried in their phones they’ve genuinely forgotten about. That part is understandable. But you also need to recognise that from his point of view, finding them feels like discovering a secret, even though it wasn’t one. He didn’t know they existed, he wasn’t meant to see them, and they show a version of you he’s never met. That’s a shock, and it’s normal for him to react emotionally.
The sex act you don’t want to do now. This is the bit that’s really hurt him. He’s not comparing himself to your ex because of the act itself, he’s comparing himself because he thinks you did something with someone else that you “wouldn’t” do with him. You need to explain that you did try it in the past, realised it wasn’t for you, and that your “no” to him wasn’t personal. It wasn’t “not with you”, it was “not anymore”.
If you want any chance of repairing things, you need to talk to him calmly and honestly. Something along the lines of:
“I understand why finding those videos upset you, and I’m really sorry you came across them. They’re from a completely different part of my life, long before I met you, and I genuinely forgot they were still there. I should have deleted them years ago and I now have.
About the sex act, I did try it in the past, and I learned it wasn’t something I enjoyed or wanted to repeat now. That doesn’t mean I wanted it with someone else and not with you. It means I didn’t want it at all. I should have explained that better instead of just saying no.
I’ve deleted the videos now because they’re not part of my life anymore. I hope we can talk this through properly, because I don’t want something from years ago to damage what we have. But I also understand you need time to process how you feel.”
This approach acknowledges his feelings without grovelling, explains your side without being defensive, and gives him a way back without pressure.
Whether he takes that path is up to him but in my mind, this is the most constructive way to open the door.