As much as the idea of Buffy and Xander having sex just seems wrong to me, I wouldn't be surprised if, at some point, say, a few days or so after the last battle in Sunnydale, "it" happened. I can the two old friends sitting together, late into the night, sharing a couple or twenty adult beverages. It starts out as, "Well, we made it. We really beat the biggest bad of them all." A few drinks later, they're drinking toasts to the ones who didn't make it, and telling funny stories about Anya and Spike. "Remember when she...?" and "I remember the time when he..."
A few drinks later, and the laughter has turned to wistfulness, then sadness, finally tears. They hold each other. both comforting and seeking comfort. A few more drinks. A chaste, sisterly kiss. Eyes meet. Hands brush tears from wet cheeks. More eye contact. Breath and pulse quickens.
Fade to black.
The next morning, two friends find themselves, hung over and in a state of serious undress, lying together under very suggestively rumpled sheets. There's a mad rush for clothes and a sprint by one or the other out the door and down the hall, and mutual hope that nne of their friends have any idea what happened the night before.
Much later, after aspirin, showers, Advil, clean clothes, Tylenol (if you've ever had a hangover, you understand) there's a phone call and a rather tentative suggestion that they meet for a private conversation about the previous night's gymnastics. Over a brunch of Alka Seltzer, strong black coffee, and Pepto Bismol, it's agreed that nothing that happened the night before actually happened. It was all just an alcohol induced mutual bad dream, even the claw marks and hickeys, which, thank god, don't show when fully dressed. And, not only did it not happen, it would never not happen again, and no one else would ever have any reason to know about.
Strong sighs of relief are followed by neither being able to meet the other's gaze for a full week or be alone together in the same room for a month.
Or probably something like that. I wouldn't be a bit surprised.