Technically the flashback you're referencing was in chapter 2. It starts on page 30, and the line you reference about not wanting kids is directly preceded by her saying "he said—" in the final panel of page 30, with the "that he didn't want kids" in the first panel of p.31.
So it is a complete thought, in-context, that leaves little room to interpret (assuming accurate translation).
You're right, chapter 2. My error there.
It continues, though. "He said that he didn't want kids, but Ryouta is my-" and gets cut off.
"But" is an important word, and every time I've been cut off at "but" it was people responding to something very different than what I was trying to say.
Knowing what we know now, emotions were running EXTREMELY high, even by the standards of divorces. They'd been trying to conceive another child so Ryouta would have a sibling, and then dad found out he's shooting blanks and never could've fathered ANY child. A whole bunch of stuff was going on all at once.
And Dad really likes being a dad, he's been raising this kid for years, and now he's being told it isn't even his kid the dumb rube, and the fight over fidelity and custody is all tangled up in a ball of sorrow and fury and regret. She left an opening to cut off any further argument about custody, and he thrust right into it.
I'm just saying that her but opens a lot of doors.