Since it was before your relationship with him it should be not a big issue, but I think he is more hurt that you were not honest with him. Or maybe the guy is someone he knows, so he is not comfortable with dating someone who that person made out with. You know men with their ego. . . . . . . . .
My boyfriend is leaving me because I did not tell him that I made out with a guy. This was before we were together and when he asked me I did not tell him. Is it justified?
Sounds kind of immature to me or just an excuse to leave you. Whatever you did before you were with him is just past history and he should know that. If he's leaving you for something like that, then I think you're better off without him.
Really doesn't matter if its justified or not, because hes going to do it either way, so to him whatever he does here is justified. You can think otherwise and try to make him stop, but its not worth it as for whatever reason, he doesn't want to be with you. Sorry. Things like this happen in young love so get used to it but time has come to just pick up the pieces and move on now. Good luck and Merry Christmas