"just think about the scene when itachi-chan got Mangekyou Sharingan the night she was with Shisui in the river."

I still don't get what you're trying to say. So she was doing him while she killed him in the river?
The requirement for mangekyou sharingan is to break the closest tie. So you would indeed expect Itachi-chan and Shisui to be going all out in the river before Itachi-chan breaks the highest level of spiritual bond with Shisui. i.e. they didn't just fuck, they made love. It makes you wonder how is it that someone capable of giving and receive the deepest form of bonding can severe such bond in cold blood to pursuit higher power. What is it that allow love and apathy to collide so strongly in the same character? Internal contrast is a sexy trait.

For this to make sense, the bond being severed should be very difficult to get. This means that Itachi-chan had been betrayed several times, and Shisui is the man that will always be on her side. Shisui would have to be the one that gave Itachi-chan hope and meaning. (just like the friendship between naruto and sasuke). So Itachi-chan is not really a slut.