IMO outside of the universe, it makes things easier for the game. Because the father simply not there makes it easier for the MC to fuck Nancy. While I dont have much problems with it in OIALT, the way the MC there gets into the pants of Carla was a bit questionable.
IN Universe, yes, it might be a stupid move. But even in universe, only because Nancy is hot and seems to be really nice does not mean that there are no issues in their relationship. But I doubt that there will be more insight on it, because most people would not want to know it anyway.
