Harsh reality but I think in most situations a partner asking about sexual fantasies from another partner should always expect the unexpected, and while it's super painful for OP to go through this and think "if I only answered differently" isn't the case. She chose to ask the questions and it's her own fault for being so significantly offended to the point of divorce, it kinda tells me that she probably would have gotten this upset about something else as well. The wife easily could have shoved that fantasy into his face like "well if you think I'll ever let you fuck someone else you're wrong" and maybe they could laugh about how OP had this dumb fantasy years later. But she wanted to be so offended that OP had discovered what a threesome was and was curious enough about it to have a fantasy. Because the reality is that if OP wanted a threesome that means he wanted to have his wife fucking someone else at the same time he was, but obviously she didn't see or understand that is how a threesome works, so if she ONLY ever wanted to fuck OP but gets offended that OP would want the two of them to fuck someone else together, than that's a her problem not OP's fault for answering truthfully. I personally have talked about threesome fantasies a few times with my gf and we've both talked about it in a hot way like we would do it, but I think deep down neither of us do want to have one because we have enough fun fucking each other but it's just hot to talk about the idea.