This is the story otherwise known as "The Role of the Sharingan in Sex and Endogamous Bonding and Incidentally Kakashi’s Massive Control Issues". I’m really not sure I can summarize it better than that. Written for the Porn Battle prompt: Kakashi/Sasuke, teacher. Porn with Characterization and Worldbuilding, I-4

Warning: clan endogamy may result in incest-y vibes for some.

Every time he bowed at just that angle that said he was a son of the senior branch; every time one of the Yasumori unthinkingly cleared the way for him in response; every time he recognized the tiny grimace that said Yasumori Koujirou really wanted to disagree with Yasumori Michiru no matter what their clan head had told them about solidarity in front of outsiders; every time he saw those things and looked by reflex for familiar eyes, eyes like his, and found only the green and hazel of Yasumori, it hurt.