Couldn't you argue since MC isn't suffering than this isn't ntr?
Yes and no, the manga absolutely do take cue from ntr, but it's in practice more of a anti-ntr story, reason I say that is because although Gotou wasn't outright suffering from it, over time he didn't want Agawa to have sex with other man, you can actually see that before the retcon, since a few times he did stop her from meeting other guys and fucked her himself.
Although I don't think the story
outright presents itself as such and no idea if it is intended to be one, in my opinion it always had some vibe that it was about Gotou "saving / fixing" Agawa from her slutty ways, with how he was always "faithful" to her, how he said he would never have sex with other women either, before the retcon prez was going to send the four great sluts to defeat him, which of course he would defeat them by not having sex, which would be "proof" of his superior pure love to Agawa.
With the retcon the story arguably abandoned its possible anti-ntr premise, which I have my suspicious is the
actual reason some people hate the retcon so much, because the hate the retcon gets sure feels similar to how some people throw a fit over ntr, though even ignoring the ntr angle, I do think some people also hate it because they did think the story was about Gotou actually saving / fixing Agawa, because of course her being a slut is baaaad.
I probably don't have much or any proof of it of course, but I do point out one thing though, ever since the story started there was always some people who bashed Agawa for being a slut, which some pretty clear signs of actual misogyny behind it, I don't know, but I'm pretty sure if we go though all comnents so far I believe we'll that got even worse after the retcon.
This isn't the first time I get that feeling by the way, one common example of what I mean is when a manga has the generic annoying lesbian side character who will obviously never win the female love interest, I can't help but notice how some people are
really overblown with their hate of such character, which makes me wonder if the closeted mmisogynists come out of the woods so they can say misogynist stuff where it's seemingly socially acceptable, since everyone hates the character.
Again it might just be me, but I do think you will notice that if you re-read the entire discussion from chapter 1.