Wow. OK, so at least now I know that he was repeating something he heard. And probably not something he meant as a direct attack aimed at me personally. It's still incredibly poor judgement, but that's much easier to navigate than the alternative.
This morning when my direct manager came into work, he immediately asked how I was feeling, and said he'd been worried about me since I escaped early the day before. We went somewhere private and I told him some things that I'd gone through during the AIDS epidemic that might bring some understanding to my emotional breakdown. Some of those I've never spoken to anyone about. I don't want to elaborate on that here, but a lot of my issues stem from survivors guilt. You think you're beyond events that happened 40 years ago and out of the blue something triggers you all over again.
He was incredibly sympathetic and completely understood how that could affect me the way it did. He's straight, but "California straight", which is to say very insightful and dismissive of anyone who isn't. I explained to him that I decided I didn't want to take this event any further. I didn't want to admonish this young man, and I really didn't want to involve upper management. He respected those wishes, but he did suggest that tomorrow (Saturday), while all of the office staff and upper management were away for the weekend, he direct myself and this young man to a private office space. He told me that if I were willing for the two of us to have a private, civil conversation and explain some of the events that I'd discussed with him, and what triggered my response to the 'joke' he told, he thought this young coworker would be sympathetic and it might be a positive lesson to learn earlier as opposed to later.
I'm not completely comfortable doing so, but I do think he's probably correct. And I don't want this to fester. As much for my own piece of mind than any other reason. We'll see how that goes, but I do suspect his instincts are good ones.