Yes, I know, I've heard about that "We've never been intimate! Except that one time..." line (which I probably misquoted like whoa there). But that is the only time we get to hear of it. To the best of my knowledge, we never hear any further details, like why and when and such, nor is there anything onscreen except a nearly throw-away line by Spike. It's still a marvelous line that I like very much, but as far as I know that's all we ever hear of it. What episode was that in, by the way? Or was it in a season 8 comic? I've never seen the context, you see, just heard that there was such a line.