I think its just some sadism-masochism play between the senpai & MC2. Maybe I don't really get what NTR means to the others saying so here, but I don't think its more than the Sado-maso play that Kuzushiro already does, just with added sexual elements. Doesn't NTR mean 'not technicially r***', which has some elements of r*** but can't fully be claimed as such? I don't think this really has any except maybe the sex part with sado-maso play.
Well whatever, I don't really care what you call it, I still love Kuzushiro's works, incl. this one.