But that's a NTS path to begin with, not a NTR one as MC needs to repeatedly and deliberately encourage both the girlfriend and the rival to hook up with each other, give them his permission to get together at several stages before they do anything. Need to tell the prowling bartender that you're not a couple, give him your permission to ask her number in the middle of your date, tell Ann she should call him after your first sexual encounter, be fine with him suddenly groping her in front of you during Joe's birthday etc. And on top of that you get all the extra dialog with Vanessa, Joe and Mystery Girl about those choices that clearly paint it as a NTS situation/MC. No NTR MC/player would make those choices, it's a sharing path for NTS fans.
You can't claim the pain/loss/grief of NTR after you deliberately put the LI in the arms of another man/NTS to begin with. It's not about the technical details of a swinging/sharing arrangement. It would be like claiming grief after playing russian roulette with your kid.