The old saying let sleeping dogs lay applies here I think. If it is something that happened in his past, so be it. Focus on the "now" instead of the "then."
I don't know why he told you in the first place. Did he want you to get angry? Does he realise that he has hurt you?
I know for one if I ever did anything like this in my past, I wouldn't be telling any of my SO's coz they don't need to know. This is exactly why he shouldn't have told you. I mean did you really need to know about his past? No of course not. What is to stop them from going off and doing it again? Now that he knows you know, he can see how angry/pissed off you get and he may end up doing it again with his friend.
The other way you could handle this also is turn this around on everyone. Ask them to show you what they did, who knows you may actually get turned on and join in.
Either way it is up to you.
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