@dojinshi Just to be clear, Rui ended things with Natsuo. Even in context, it's still really dumb, but she didn't really give Natsuo a choice.
For others like dojinshi who haven't kept up, here's the build-up:
Rui is preggers
Togen-sensei dies, gives Natsuo his unfinished manuscript
Natsuo finishes the manuscript to wide praise, proposes to Rui
Paparazzi tries to expose Natsuo's sister-fucking ways
Hina exposes paparazzi's unethical methods to his boss and gets him fired before article is published
Paparazzi runs down Hina in his car, Hina goes comatose
Rui tells Natsuo they can't wed, Hina's love was too stronk
Timeskip, kid is born, Natsuo and Rui live together caring for Hina
Rui tells Natsuo to marry Hina
Hina comes out of her coma, recovers over a second time skip
Marriage
That's like 35-40 chapters of stuff so obvs there was more nuance but not enough to make this ending feel good.