Of course if it was a colossal turn-off for you, and you'd already told him you had no interest in receiving explicit photos, YANBU to dump him.
Personally, I don't really understand the viscerally horrified reaction that people are having on this thread to the mere idea of receiving pictures/video of someone's dick when it's a dick you're already intimately acquainted with. For me, if I'm happy to get up close and personal with it in real life, I'm more than happy to see it in pictures. And outside the realms of Mumsnet, lots of women (including me) do like watching their partner touch himself. It's also a very popular category of porn for women.
I do find it interesting that everyone is saying 'Eww, gross, disgusting, what woman would ever want to receive a pic/video of their boyfriend wanking' because I'd really like receiving that from someone, providing I was already in a sexual relationship with them, and I definitely know plenty of other women who would too. I wouldn't send them pics or video in return, but I'd happily receive them.
Do the people who find it gross to see that in a video also find it gross to see their partner touch themselves in real life? Would you never want him watch him do that in the bedroom either? (Genuine question, no judgement.)
Of course I'm not saying you (or any of the other people on the thread) are wrong - each to their own and you are completely entitled to feel the way you feel. And of course he should never have done it if you'd specifically told him that wasn't something you were into. I guess I just hadn't realised it was something so many people would be shocked about.