It wasn't as though I had said, "You're a man now? Well, we're done then. Don't let the door hit you on the way out."

That's probably how it felt to him. It would almost certainly come across as "you're transitioning--we're done and I'm moving out," which comes across as really judgmental. I get it, you're not attracted to men, that's perfectly legitimate. But it seems like your very first focus of attention in what was a very vulnerable moment for him was whether or not you still wanted to date him, which is painfully you-focused in that moment. And yeah, it might have been a little better to focus more on what he was telling you in that moment. It seems like a very slight difference, but it's a really important difference. You kind of made his coming out to you into a you-moment.