if i understand it correctly the ntr/sharing path of sophia involves his son being part of it is it (i didnt play ntr, i dont like it so i dont know)? the non-ntr part is just him watching while the mc have sex with his mom... moreover, while i follow the walkthrough when i finally get to the part where sophias son is asking the mc for him to join, there are no choices showing up... it just keeps on going until mc have sex with sophia while her son watched.
if there is really a choice i would go back even just to deny him, but im sure the walkthrough or the game does not say whatever...