The cheating is still at the expense of the other protagonist though. Even though you control the other protagonist making that decision, let's say in this case Lena cucking Ian, Ian - a protagonist, still gets his love interest cheating on him. By changing perspective and controlling Ian, the point of view of the player still follows a protagonist being cucked by a love interest. Now that perspective may change again and we get to have Lena's point of view so when Lena cucks Ian it's only cheating. In other words given that this game has multiple protagonists, when we are in Lena's perspective, we are not experiencing ntr when she is cheating, but as we shift our pov and we get to see and control Ian, when Lena cheated on Ian, we get to experience ntr from a love interest cheating on the player-character, which in this case is Ian. I only used Ian getting cheated as an example. The roles could reverse and Lena could be the one getting cheated.