MC was retarded. After all that, he could at least say that there may be a good possibility in the right situation down the road, and that may have appeased her. And honestly that even might be the truth, if in the right situation where they can kiss or reproduce in a safe manner where they can either control the newborns that come out and make them on the "good" side, wouldn't that be an ok situation to kiss then? MC instead literally gave a no to her through lying, and she saw right through it.