I love fic where they cheat on Zak (yours is probably one of my faves), but canonically, it is really hard for me to believe that they had sex while Zak was alive. Of course, there were a lot of repressed feelings when they met two years after Zak's death, but I always got the impresssion that everything between them was pretty much platonic (For instance, I always had the vibe that their kiss in "Home" was actually their first kiss --we know that it truly wasn't, but still...).