Given that the original comics (before he made this series) was a straight up BDSM relationship, I don't think its so much the MC being weak as much as the MC being the sub. Given how Nagotoro herself is heavily implied to have a masochistic streak herself, and more emotional vulnerable than she lets on (she basically shuts down in the first comic when paisen refuses to hold her hand), I'd say that its a matter of two sides of the same coin, only paisen's an introvert and nagotoro is an extrovert.