Well... in comparison with Japanese theatrical movie, isn't this kind of ambiguous ending nothing out of the usual ?
At least now he knew about her identity. I hope he would tread it carefully, especially because he had a history with her parents. He also know she is underage.
But if he believe she could finally pull him out, maybe it's a risk worth for him to take.
There's still the question if he would go to his father's path, loved by girls but finding himself unable to love them back.
Nao surely had the problem of his father's mistresses, loving him blindly.
It would be interesting to know if she also realize he is that handsome uncle who visited home when she is younger.
how the F did Sa-chan meet her husband
@GuitarLegend : Isn't Sa-chan's husband that guy who is close to her in university ?
Maybe she met him again years later, and finally get together.