Netorare = 寝取られ = partner stealing. Very literal and direct translation. Nothing to do with sharing.
And I don't believe Chika is shared anywhere while dating anyone, if that's what you were implying.
NTR is also NeToRi and NeToRase.
Being a whore while being an L.I involved with an M.C makes it shared.
Netorase is about the only one that fit the bill. Bit that's not what the majority of people are referring to when they mention NTR.
So please explain to me how the LI is involved with the MC?
