Probably not it, but in my "Seed of Doubt," Lois and Clark both succumb to the Revenge potion before they are a couple and wind up sleeping together. Later, Lois is staying at Clark's place (dating but not sleeping together, Lois is pregnant with either his or Luthor's kid) and Clark is called away in the middle of the night. Lois wakes up to find him missing and waits for him to get back, then confronts him. No introspection piece, which leads me to guess this probably isn't the fic you're looking for, but just in case, I figured I'd throw that out there.
