If 75% of the "choices " in the game are "Do you want to progress down the path of fucking the step daughter or do you want to reject her". Then guess what the game by definition is about fucking the step-daughter, even if you opt out. Also the description of the game literally includes " Will the main character succumb to temptation or remain a faithful husband? It's up to you to decide. " AKA the focus of the game is whether or not you fuck the step-daughter.
Just because your being weird and playing the role of a faithful husband, in a game where the intended way to play is to cheat on her. Doesn't magically mean the game isn't about cheating. As further evidence, do you get extra scenes by choosing not to fuck the step-daughter, or stay faithful in the few instances you have the option of fucking other people? No you don't, that tells us your not intended to play that way.