Having finally gotten to play the Kagura Games version of this, I can say with confident that is probably one of the first True NTR games. And here's why.
You control Keita and have a somewhat passive influence over Ryoka. Whereas in most NTR games you play as the female character doing the NTR. So you have control over who she NTRs with. This game, not so much.
So basically, not only is Keita getting NTRed, but you the player are also being NTRed since you don't control Ryoka.
And as far as I could tell, I couldn't stop Ryoka from constantly going to Oyami, lol. All you can do is watch as Ryoka slowly drifts further away from Keita.
There also doesn't appear to many activities other than playing video games that Keita and Ryoka can do together.
Well, regardless. I get it now. I get why people don't like NTR.