actually, it doesn't make zero sense. I'm a woman as well, and I would much agree with the original comment, and other women have said the same to me that they wouldn't move with a man that is not their husband. I'm not sure proposing for the sake of getting her to move with OP is the right intention, but if he feels serious enough about the girl then I'd say do it.

Unfortunately unlike men, it tends to be more difficult for women to find serious relationships when they're older, women only have limited amount of time of being able to reproduce. So even if women are not actively thinking about this, it is instinctive to only take the sacrifice of major life changes when we are able to tell there's a commitment to a relationship.