I am finding it hard to believe OP, that so few people replying to you seem to understand the hurt and embarrassment that he caused you by not having previously told you about the status of your now mutual friend. But in case it was your explanation that most of them didn't understand - although it was very clear - please bear with me while I try to explain this part of the situation to any of the pp's who are prepared to read it:
You meet someone. You eventually fall in love with each other. You may or may not be, the sort of couple who shares with each other the details (I don't mean in depth details of their actual sex lives, but whether or not it was a sexual relationship) of previous partners.
However, once you and your partner are in a serious relationship, if either of you meet an ex, either accidentally or through an arranged meeting, and whether your new partner is with you at the time or not, you or your partner, (whichever one is relevant) should tell each other about the relationship you or they had with your (or their) ex.
For example:
You the female reader are attending a party with your partner, and to mix it up, it is you who hadn't already told your male, serious partner, about your previous partner. What you should do...
"Oh Hi John, long time no see, please let me introduce you to my partner David, David, this is my ex John, we met at Uni and went out for about 6 months before he realised that he and Jane had much more in common than he and I did (muted laughter all around)! By the way John, is Jane here with you tonight?"
What you should NOT do - again, you the female reader are there with your newish, but serious partner..
"Hi John, long time no see, please let me introduce you to my partner David, David, this is John, a friend from Uni. Is Jane here with you tonight? ..... Oh, you split up but now you are engaged to Flo, is she here tonight as I would love to meet her?"
3 Years later (after the 2nd example), when your partner David has spent lots of time with your ex John, sometimes as a foursome (not sexual!) with you the reader - Elsie, and Flo, but sometimes just David with John, as the 2 of them have formed a friendship of their own:
Flo talking to David...
"Has Elsie ever told you about the time that she and John went for a romantic weekend in Amsterdam, but they got lost late on the first night after a few drinks and a couple of splivs, and the only people they could see were a couple of scantily clad ladies sitting behind 2 seperate windows (John and Elsie were very young, and even more naive - when it came to the other thing Amsterdam was famous for!), so John went and knocked on one of the windows to ask for directions..."
Flo gives a loud guffaw at this point.
As I said before, if most of the pp's on here were David (or you OP) do they honestly mean that they would not have been hurt or embarrassed by the fact that they had been friends with one of Elsie's exes, and had not known for circa 3 years that John had had an intimate relationship with Elsie at some point?
I would have been bloody fuming, and like you OP I wouldn't have given a rats arse about them previously having been in a relationship together, my BIG problem would be that I was the ONLY ONE who didn't know about it!
I am not surprised that you lashed out when you were feeling so hurt OP, and yes, as you have already admitted, it was very unfortunate that you said the one thing that probably caused him the most pain - but it was done when you were under a lot of duress. You are obviously much too lovely a person to do that sort of thing in cold-blood, so I think you should cut yourself some slack, and I think the pp's should too.
As for your partner, it sounds like he quite often laughs cruelly at you to purposely try to make you feel small. Maybe this worm has turned, and either he learns to stop being so needlessly and horribly cruel, or maybe you will begin to see him in a new light. He has treated you shabbily, he doesn't sound t all remorseful, maybe YOU should be re-thinking your relationship. You deserve someone as loyal and honest and kind as you are, it doesn't sound to me like he comes anywhere near to fitting that bill!
ps. Having now read everything I just wrote, I have come to the conclusion that you were much easier to understand in the first place OP!