Extremely niche and specific rant no one cares about incoming...
But cuckoldry is not adultery! By the description, the main conceit of this game is consensual among all three parties. That's not adultery. That's just an arrangement among consenting adults. Adultery began as a legally punishable crime (and still is, according to some crazy laws still in existence). It was something objectionable from a moral standpoint and necessarily done covertly. If everyone involved is knowing, willing, and into it, that's just kink. Completely different.
As an aside, i would say this probably isn't even NTR, but at least it's not tagged as cheating. That's when this miscategorization really gets annoying.