"Is bound to be unhappy"
There is an oxymoron in this. It would need to be seriously fucked up situation you are in for you to be constantly unhappy.
Sure, it may have more unhappy moments than not, but having children, watch them grow up and become independent... she might never get happy with her partner, but at the very least, there are good moments that are born from it.
If he ever actually becomes her real partner I will learn proper Japanese just so I can send a scathing letter to the author, criticising that the many lifetimes of torture they (the glasses guy) has put the girl through will make it 100% impossible for her to forgive him.
There's a point where forgiving someone is impossible, and that man has crossed it too way extreme ends.