Yeah i gotta agree, everything the gf does is without consent even if the intial date/meeting was consented upon. Then as it progresses, its almost begrudging consent from MC. There's a fine line between NTS and NTR and this is definitely more NTR. And the sudden emotional flip flop of the GF is wild.
Not to get too deep into the weeds (but I am going to anyway

) - this is NTR, and dark NTR at that. While I see people commenting on the MC's uncertainty with his fetish, the girlfriend going out with and fucking around with another man is just cheating regardless of whether the MC is investigating his fetish or not. Plainly put, the MC is the definition of a 'beta-cuck' without any prior knowledge of what is happening until after the fact. Taking into consideration that he is at odds with this feeling of love for her, makes me think he should also investigate
'how to confuse sex for love and deny it is happening even with evidence to the contrary' The MC continues to talk about how much she loves him yet she gives no real indication that she even considers his feelings, let alone has any sense of love for him. His love for her is a matter of convenience and because he is weak-willed and confused, he is allowing this all to happen.
I have no issue with and completely understand the difference between the emotional aspect of love and the physical aspect of sex...I can also separate the two and our MC seems to be struggling with that...at least in part. I can't tell if this is cognative dissonance or if he is just so weak as an ego that he is willing to put up with something so drastic while he is all but being pushed to accept a fetish he isn't even sure he is into. Either way the brutal nature of how it happens is a sure way to turn off most people that want to explore Netorase or Netorare.
If you are into Netorare and like the idea of your love interest being taken by a someone and enjoy the intense feelings of jealousy, betrayal, etc. then this is right up your alley. IF, however, you prefer a story about a couple where the MC is unsure about his fetish and the love interest starts pushing for more to help him along that path, then this one pushes far too much, too fast and not really a good portrayal of anything except a cock-hungry whore, with no self-restraint, and little to no consideration for her partner's feelings. She is using him, period - and if that is what you are into, all good. I am not a fan. I prefer the mutual aspects of NTR, even when it ends up being Netorare, as long as there is an understanding that both partners will get something they want out of it before someone starts cheating.