They are not together. MC and Leah are not in a relationship. If Leah then has sex with someone else, it's not cheating, it's not sharing, it's not cucking, and it's certainly not NTR. This site, or at least a very vocal minority on it, thinks that any person in a relationship with anybody other than the main character is NTR and that's bullshit.
The main character has chosen Avalon, or Avalon and Dallas, or Avalon and that other girl whose route I still haven't played. He has not chosen Leah (at least not yet).