Early in our relationship Dp and his Ex exchanged explicit messages and pictures. He had the perfect excuse of wanting to keep her happy so that he could see the children, that he knew he'd gone too far, he'd stopped and didn't expect me to find out. She helpfully let me know through screenshots of emails. It was a long time ago, I thought I had managed to put it out of my mind. Some things didn't add up, she mentioned Skype calls, he said he hadn't done anything on them, but sometimes it was easier to talk to his child on video than over the phone, but he swore blind he had no interest in his ex.
I've had an anonymous email. He doesn't know about the email as I need to sort my head out and work out what to do. There are screenshots of messages between the two of them within the last week, where His Ex has been bringing up that during the period of messages, they used to Skype and that they would both 'get off' on asking each other to do things. He has ignored any mention of this in replies and only responded to bits concerning his children. The email I've received says that there are recordings of the Skypes between them.
It was years ago, but I still feel devastated, and stupid that I didn't leave then and there. The timing of the email is perfect, we are about to get married. I don't know what to do.