I always thought that, just maybe, Xander and Cordy did finally have sex right after the final scene of Part 2 of "Graduation Day." Right after Oz says, "No, we survived GRADUATION," and they turn and walk away, Cordy is speaking to Xander, and it looks like she puts her hand on his arm and laughs. By then, she had had enough time for the sting of his betrayal to have diminished a bit, and she had to know that he was genuinely sorry about what happened. (I don't think that him paying for her prom dress had anything to do with it, by the way. Cordy wasn't a hooker. Still, she would have seen it as a sweet gesture) It's not uncommon for teenagers to lose their virginity after Prom or Graduation, I do honestly believe that Cordy was a virgin at that time, and I think that that's pretty much what happened between Xander and Cordy. She was leaving Sunnydale to seek her fortune in the Big City, and I don't think that she would have wanted to go away completely inexperienced. Maybe she no longer thought that she was in love with him, but she felt genuine affection for him, and I think that, under those circumstances, that would have been enough.