Because of main theme of the story....it's based on who is Mr X and why he want to destroy the relationship. Maybe to reveal that part, Clara needed to be corrupted or she should become cheating wife as a twist where irrespective of her faithfulness towards Alan, she found a way to satisfy her sexual needs from someone else because Alan cant do that.So why does the creator waste time and resources on a false path of faithfulness that leads nowhere? should he immediately focus on the path of corruption?
Same with perfect marriage..the main theme is whether kendra will be caught or not/ whether David will cheat really or not/ Whether Ana will get the truth about David's cheating or not/ whether David will forgive Ana after knowing she cheated out of anger or Ana will go for divorce....this is main theme doesnt matter if developer want Ana to be fucked by everyone or not. That is main path of story.
Here in corrupted heart whether Ana/alan will get along or not...Whether alan will give in to Anna seduction or not/X has some background story to ruin marriage or not...and why X want Clara to cheat / will there be path for forgiveness or not because Alan told Clara to give away her dignity to make the mission successful ??? This should be the main theme...Showing sex / cheating sex should not be the main plot of the story. Creating hype for whether to cheat or not will ruin the main theme about the story.