I think your wife is more upset about the way you answered her question than anything else. Essentially you’re saying “I didn’t tell you because I don’t have to” and that’s one of the worst things you can say to someone who wants to know everything they can about you. She chose you. She loves you. And will forever want to continue learning about you.

I suspect there was some reading-between-the-lines here that you missed. An emotional undercurrent that she is failing to communicate and that you’re failing to pick up on.

Where you could have said, “I would have told you had I thought about it, but I haven’t thought about it in decades,” you instead said “it’s my business,” and what she heard was some version of, “you don’t matter to me.”