I posted this as a comment and thought to share my current theory with the broader JJK subreddit.

My theory: My head cannon is that Jin and Kaori Itadori became desperate to have a kid, and that she may have lost children in the process of pregnancy or pregnancies. Then Kenjaku may have promised to fulfill their wishes by making a binding vow with the two of them; while possibly hiding his agenda. Meaning that Jin, or possibly both, knew he was making a "deal with the devil" to get what he wanted no matter what.

Alternatively, Jin may have had a psychotic episode after her death that led him to believe Kaori is still alive, not noticing that Kenjaku took control of her body.

I'm also almost certain that all the itadori's were Jujutsu Sorcerers, and that Wasuke Itadori, Yuji's grandpa, has forsaken his Cursed energy, via a binding vow, to protect Yuji from the world of curses (i.e. Jujutsu Society and curses) until a certain point. Either a specific date or upon his death. Because on the day of Wasuke's death was the day Yuji saw curses for the first time. So Wasuke probably knew bad things were coming for his grandson and wanted to protect him at all costs, whilst wanting Yuji to stay true to himself, thus, cursing him with helping others on his deathbed.

Also remember, at no point in time was Yuji ever a normal person. He has been special ever since his birth. Not one person alive chucks a shotput like a baseball. That thing would have kept going if it didn't hit the soccer goals metal rim. Which he dinted by the way.