Sure, it definitely gets out of hand with some people, but that doesn't really apply here. This isn't some girl who dated someone else once before the the MC turned. This is the guys wife, who was he dating her when it happened. And the long term consequences are still part of the story, in Mao.
If it happened to literally any other girl from the flashbacks, I'd agree that it doesn't qualify, but Hiroko at the very least fits into a grey area, due to her relationship with Matsu, and the existence of Mao.
This sites definition also doesn't matter, as they aren't the ones decided what goes into this game. When BlueCat says "no NTR" he's using his own definition, accurate or not.
Regardless of any definition though, as that's not really what we were talking about, it touches the subject in a close enough way, so I don't think anyone but Matus can be her dad.