I'm curious about the "you" at the end. I assume that Naoto addressed Nagatoro with
お前 and she picked up on that, which harkens back to early on in the manga (chapter 3, p. 5). Then, it was clear he was trying to push back against Nagatoro's teasing at the time, trying to assert himself.
This is interesting (I've actively noticed this writing technique in another manga, though I'm sure it's not uncommon): their usual dynamic, and its history, is being recontextualized in the context of them being in an intimate relationship. Earlier in their relationship, Nagatoro invading Naoto's privacy would ironically have been a minor deal because she insisted on being around him; on his end, not only did he lack the mental fortitude to push her away, but he also wanted her around, and so he tolerated her antics (he still resisted, but without coming close to cutting her off).
Also earlier in their relationship, Naoto concealing this kind of truth would have been a minor deal: not only were they not dating and who he was involved with wouldn't necessarily be his business, but Nagatoro would have been more aware that she was being invasive (and doing something wrong), because that
was part of her being a nuisance, and her being a nuisance was both sword and shield: to simultaneously protect herself emotionally and establish a place in the life of someone she took interest in. Even some time before they started going out properly, when she was letting her guard down and their dynamic became their "normal", she may have forgotten that-- she may have never even realized that was exactly what was going on.
I can't at all begrudge anyone for getting hung up on the lack of openness and even trying to figure out how to apportion blame, but there's something more foundational at hand, and I'm expecting for that to be explored alongside-- if not more than-- "who's most in the wrong" (certainly, these two won't be putting stock in that calculus).
I don't think they're wrong, especially if we're dealing with a story that's no stranger to conflict.
I've become increasingly certain that consumers don't have a complete awareness of what they continue to give attention to. Obviously, people give positive reactions to what they like. There are also people who gaslight themselves into believing they like a certain work or writing sequence despite having no such intrinsic interest, because they want to fit in or peacock about how "mature" and "cool" they are.
And, most pertinently, there are people who do something like the inverse of that, where they rag incessantly about something they willingly continue to consume, for the same reasons.
Nagatoro knew (or should have known) this was coming, since she was present when the bet was made.
Not only that, but Nagatoro did this while Naoto wasn't watching, specifically after he was occupied and couldn't see her. She snuck around, herself.