Is your exH going to be there again? Since that was the trigger last time, it seems crucial to know that.
I agree with the idea that it seems a bit disloyal to just accept your DP being excluded. The idea that couples 'come as a package' can be taken too far - everyone knows someone who drags their other half along to what should be girls' gossipy chats etc. But functions (ie weddings, christenings, family parties) are when I would always expect an established couple (married, living together, or together for a good few years) to both be invited. I know that can be financially tough but it's the price you have to pay for acknowledging that your guests have lives of their own and that, while it's your day, it is nice to be able to give some thought to your guests enjoying it too.
I echo the poster who asked why you don't challenge your friend - however politely and kindly - about this. I would phone (not email, this is something where you need to avoid misinterpretation) and say
'You know how much I would love to be there for your wedding day. The one problem is this issue about DP not being able to come because of (previous occasion) because that makes me uncomfortable. I know I'm obviously going to see this differently to you, but in my view it was a one-off thing caused by the stress of meeting exH. I can assure you he hasn't done this at any other time, and I'd really like you and (other friends) to get to know him for the great bloke he is. So, I wanted to ask you if you'll reconsider. If you want to think about it and tell me in a few days' time, that's OK.'
Then wait and see what he says. If he has a change of heart, great, all systems go! If he still says no, I would regretfully decline and suggest some other time when you can catch up. But I would also then send a card etc. so that you are being the bigger person. Maybe then in time they will realise they are on their moral high horse a bit over this. And if this is the outcome, I would just not mention it to your DP - assuming that he is genuinely sorry and embarrassed about how he was last time, because then there's no point in punishing him with it again (the punishment being on their part, not yours, I mean).