I can only agree with this game not having the NTR flag in the traditional way. If anything it could be seen as a partially Netorase-ish game. The big difference is that the cuck role is not for the MC but for the girlfriend, and it's agreed by both of them, so you can question if you really can call it cuck. The girlfriend agrees and the MC (very) reluctantly agrees. Both know, however, that it's for a big part role playing, and their relation is not in actual danger. Sure - The MC finally develops some feelings for the third party (after denying that in a such infuriating stupid way that you want to strangle him most of the time), but at "worst" it would turn into a triangular relationship, because the girlfriend likes that other person too.
This game would be a perfect game in my eyes, if the MC would not behave so reluctant to the whole idea, and would treat the third person with more respect/love. The way it is now, you want to kick that stupid MC in the ass several times during the game for his shitty behaviour against the third party.