If the character is seeing a girl fuck someone else and isn't being forced to watch/getting yo girl stole/etc, then
Female exhibitionism and/or
male voyeurism spring to mind. Why would it be any different than watching her masturbate? The overuse of NTR and cuckold these days is going to utterly ruin the genre. As it is, a good number of games with NTR tags that I've played here (and other places) barely touch on the core tenants of the fetish.
True NTR, to me, is a scenario like this:
Now, not all NTR has to be like that, of course, but there should be at least a
few similarities. To me, it's the mental/emotional damage of NTR that "makes" it, not the action/s.
15th edit: I keep forgetting to praise this game amidst this genre discussion and drama. For 0.2 this is quite impressive and shows a lot of promise. I will definitely be following this title. I'm especially fond of the anatomical.. "correctness" of the models, for lack of better phrasing. I'm already a fan of fit girls, so my bias is heavily skewed against many erogame girls anyway.. but man I tire of the "watermelon tits on a petite frame" trope which seems so, so common in erogames anymore. It immediately breaks my suspension of disbelief.