Buffy/Giles/Willow/Xander - Post-Chosen, naked hot-tubbing leads to more. Extra love if a little game of Anywhere But Here or I Never leads to disclosures and sexiness. Alcoholic beverages wouldn't hurt, either.
Giles would never harness magic or demons or group sex for his own pleasure again. But he still has fantasies, that he plays out in the safety of his mind. They involve Xander. And maybe some of that other stuff.
Giles/Buffy - power games, with the Watcher in control.
Giles/Ethan - after Grave, in England, Giles needs his oldest friend and some sexual healing.
Giles/Jenny - School sex - can be sexiness, as opposed to actual sex, but the line where carried-away overtakes discretion is what I was thinking.
Giles/Joyce, post "Band Candy", "About that police car..."
Giles/Xander the night before Sunnydale falls. Xander needs to feel good and Giles needs something worth surviving for.
Xander/OFC with Xander/Giles or Giles mentoring Xander - Xander is attracted to his slayer.
You've oppressed me by omitting my favorite prompt.
Compared to other fandoms' kink memes, this one isn't very kinky, is it?