I've previously said that it's difficult to watch since I'm heavily invested in these characters, but I wouldn't go so far as to describe it as NTR. For me, NTR is about when the protagonist is deliberately forced to watch a LI get fucked in front of them to torture them, and it's most especially NTR if the LI is the one who wants to do it as a way of flaunting it in the protag's face.
The M&J scene, for me, is more like watching a sex tape of your girlfriend with her ex; you accept that she's had relationships before but you don't want to see it because you'd rather not know what their previous relationship was like.
Corruption is basically when you take someone with little to no sexual experience and turn them into a totally cock hungry slut who will do pretty much anything. But this can also be linked to NTR if making the wrong choices with a LI can lead them down a corruption path. Case-in-point would be a game like Good Girl Gone Bad, which when I played the first time saw the friend Eva become a total whore and it wasn't pleasant to see, but making better decisions didn't lead her down that path.
Oh boy, I don't know if I like the idea of double relationship drama with a LI-centric DnG game AND a birthday party with all the LIs in attendance in one episode.
Hopefully it won't be, but I can't really see a scenario where the MC doesn't have invite all the LIs to celebrate his birthday.