@Weasalopes
We are meaning NTR as infidelity. While I think netorare technically refers specifically to "stealing away" the girl, as opposed to one-off cheating or agreed-upon cuckolding, it's frequently used as a shorthand for general sexual/romantic infidelity in all forms, especially by those of us who hate it all and don't care much to distinguish. If you keep in mind why she's doing what she's doing, infidelity seems like a very obvious topic that comes up in the sort of material she takes reference from, so it's something that might easily come into the story.
In chapter 1 it was already briefly suggested that she might have tried to look for someone else if he hadn't agreed to it. While she did go on to say "Like hell I could ask anyone else", and I hope that was intended by the author to shoot down the idea from the get-go, it's still a very real possibility for this kind of situation, especially as things progress and we have a combination of i) increased confidence, ii) broader and possibly more extreme interests, and iii) emotional confusion and potential drama once one (or both) of them inevitably starts thinking about being more than "playmates".
It is related to the idea of them not knowing their boundaries though, since that's one of the biggest and most ruinous lines you can cross. That's why, even though this chapter didn't do anything suggestive of NTR directly, it still got brought up in the comments, because poorly defined limits are the kind of situation where it thrives and ruins everything.
I don't even think "research" is necessary. Just a bit of common sense. Whenever I see stories like this I can't help ask myself why on Earth the MCs didn't sit down before they did anything and say to each other "here's what we're doing, here's what we're never doing, and here's how we keep things under control". That seems obvious to me and I'm just a volcel who's never held a hand, so I can't understand why so many manga MCs don't think of it, if not from the beginning then at least once they've gone home and reflected after the first session.