A mixture of Voyeurism, presenting her more sexual fun than a single man can give (let´s face it, no matter how good we are in bed, there comes a time our "best friend" cannot get up again that evening), proud to have such a great girlfriend/wife, happiness she has fun and being wild/slutty, getting hot for reclaiming her, taking her back.Let's drop for a moment the quesstion of definitions and of the games' direction. I'm above all interested in the psychology behind those kinks and their proper appreciation.
So if I may ask, taking the situation in the game as example: the MC is obviously nudging is gf to go forward with other guys. You are apprently interpeting that in the paradgim of a "balanced of stag/vixen" path. So: what exactly is his motivation in that very moment in your perception, what does he get out of it watching his girlfriend doing that? Because I have a feeling, that here starts the difference to what I (and others) call NTS, in this motivations and feelings.
It is about having kinky sexual fun together, both being fully ok with what goes on. Being clear what can be done and what is a red line not to be crossed. Humiliation and denial have no place here, that is something which invariably sends a relationship on the wrong course over time, because of the inherent discordance.