Well "infidelity" is the action of being unfaithful to a spouse or partner. At that point u have fooled around a lil (kissed, grouped & bj) but you're not actualy in a relationship with her yet. She is horny & (technically) singal when u lick & mount her as the wolf. After THIS scene u & Mia start working 2wards an actual relationship & secretly start dating behind her grannies back. After u knock her up (& u take responsibility for the child) she asks her granny if u can live with them. Trust me man u r missing out on one of the sweetest girls in the game if u let that one scene that isnt fully in her power (MC is manipulative) ruin that for u.
But at the end of the day, no skin off my teeth, it just means u have less scenes, quests ect... to play through. U do u bro.