I mean, that's the style Fujino has gone to with his previous games, the "MC remains completely ignorant of being ultra-NTR-ed", that's all, he's just keeping tradition XP Hell, the fact that there IS an Ending where the MC discovers it (But does nothing because he enjoys it) is completely new for him period.
Yeah, I know, which is it's so stupid, that way of thinking fits Liz even less than her trying to separate Kirito and Asuna for her own benefit in the first place, as I said at least Suguha was trying to take advantage of the chances she got in front of her XD
She clearly doesn't know of the wisdom of your signature's phrase