Well, as long as the MC is in the know that that is something that can happen and is okay with that? Nts clearly. Now, if she never tells him afterwards or even lies then it goes into ntr territory, even if she was allowed to do it without asking. But as long as everything is open without lies? Yeah, nts. It'a all about consent after all.
it's hard to understand a border of NTR and NTS...
I think for me the NTS game is when, everything that can happen can be cancelled, after all.
Personally, I chose without surprises. For me, it's preferable to have no surprises at all, knowing everything...
Any unsaid things or lies are already perceived as betrayal, so it's more of NTR.
But in general, if you agreed to surprises, then you should be prepared for her to come with creampied pussy or even pregnant. If you accuse her that this is too much, then this is more of a betrayal on your part, because you gave her freedom of choice.
If you agreed to surprises, and she came pregnant, it's more of NTS, the main thing is that she returned to her own boyfriend and honestly said who the father was...
Of course, she should take into account the interests of her boyfriend. But discussing the boundaries of surprises is like not surprises at all...
Well, for example, it's reasonable that before she comes pregnant from someone else, she needs to understand how much her boyfriend is ready to live with a child from another guy.