So... even if the act of sex doesn't hurt the MC it still counts as Netorare/Netori?
There isn't a simple answer to this because NTR is an extremely loaded term that is used very loosely despite people taking whatever it means all too seriously.
In a technical sense NTR is the shorthand for Netorare, a Japanese term that is usually translated as cuckoldry/infidelity but my understanding is that it's more precisely the sense of emotional anguish a person (our POV character) feels when their sexual partner is stolen from them by a third party. It's not just about the partner having sex with a third party, it's about the partner coming to prefer the third party to our MC. By the same token, the partner has to be someone with an established relationship with the MC (wife/husband or formal girlfriend/boyfriend) before losing him/her can count as NTR; stealing a fuck buddy or platonic friend doesn't pack the punch needed to qualify.
Of course the strict technical sense is quickly muddied because NTR can also be used to abbreviate other Japanese terms like Netorase (consensually sharing your partner with third parties) or Netori (when the MC seduces a third parties' sexual partner). Those terms still revolve around sex with non-partners, but they have wildly different connotations. So NTR as a term can mean different things and the context isn't always clear.
Thus when it comes to talking about whether a game has NTR in it, I find it's more helpful to lay out the different types of extra-relationship sex that might be present in a game and let the audience draw its own conclusions on where they fall in the NTR spectrum. It's slow and tedious, but it avoids a lot of circular arguments.
There is another problem, however, because NTR is such a broadly applicable and emotionally charged term, and because the vast majority of AVN MCs (and players) are male, people on forums like these tend to use NTR very broadly as an all-purpose negative epithet for
anything involving other male characters having sex with female characters. In this case it doesn't matter what the relationship between the characters is or whether both parties consent to the activity - or even if the activity is shown rather than merely implied - as soon as a second penis is involved it's
NTR!!
Obviously when the term is used in this sense, there's very little point laying out particulars; the practical upshot is that the situation in question is exceedingly unpleasant to the person describing it as NTR. What matters here, IMHO, is whether the situation labeled as 'NTR' is central to the game's premise. If it is, this usually indicates that the game is never going to be that players' cup of tea; the game is
about material the player hates, so isolating it certain paths or having it happen off-camera wouldn't help. If the situation is more of a one-off event on a minor side story, then it may be something the player could live with if it was kept off-camera or happened only on branches they never choose.
To bring this back to
Chasing Sunsets, the game doesn't have any Netorare in it: there's no risk of anyone stealing a girl from the MC. There are aspects of Netori with Fiona and Linda; the former is unavoidable but largely skippable after the prologue, the latter is part of the MC's backstory but we're assured it did no harm and it will only reoccur if we choose to rekindle the affair. We're just starting to see elements of Netorase with the Mallory/Amanda threesome, though it's a bit hamfisted. Soo far it will only happens if we choose to permit it. Presumably the much teased Jaye/Mallory throuple would be similarly avoidable if it ever materializes. So all told the game doesn't have much claim to NTR in a technical sense.
On the more figurative sense of NTR as 'other characters messing with my girls!,' there are three potential blips on the radar. The first is Christian's 'incident' with Jaye in the prologue. The second is Jaye's implied first time having sex while the MC is on walkabout. The third is Mallory begging the MC to include Amanda in the sex scene in Chapter 7.
For Christian, the game implies at first that he did 'something' with Jaye in the prologue, though we later learn he was stopped before anything explicit happened. On top of that, while Christian doesn't *steal* Jaye, she certainly uses him to make the MC jealous and he indirectly stokes much of the emotional turmoil between the two that has poisoned their relationship at the start of the game. I can see how that might bother some players, and it's pretty central to the story so there's no getting around it. Still, it's mostly a misunderstanding blown all out of proportion, we get a chance for cathartic revenge on Christian, and even in the midst of the misunderstanding it's obvious that Jaye is far more invested in the MC than she ever was in Christian.
The second case is an interesting one since it's so far offscreen it's entirely possible to miss it and never know it existed. Essentially if you failed to max out Jaye's points in the prologue, it's implied in Chapter 4 (and again in chapter 6) that Jaye lost her virginity at some point while the MC had run away from home and traveled the world with David; it you maxed out her points Jaye strongly implies she's still a virgin (lesbian sex with Tara notwithstanding). This is a pretty clear case of other dicks getting involved in the MC's territory, but it happens very far off-screen. It's also technically within the players control, though only indirectly (and the mechanism is extremely opaque). Personally, I think it's a good thing when characters have lives (even sex lives) outside the MC - it makes the world feel bigger and more dynamic - so this doesn't bother me. But I can accept that hardcore devotees of the MC-centric worldview exist and I see why this might bother them given the way they're likely to encounter it (i.e. punching Christian for messing with 'their' girl). I wouldn't call it NTR, though.
The third case is a rare instance of a female character seeming to steal a LI's affection. To be clear, that doesn't actually happen: Mallory
asks to involve Amanda, but she unambiguously accepts the MC's decision if he declines. Still, the revelation that there's another character Mallory might *almost* leave the MC for is a big deal, especially when the scene is framed as more about Amanda/Mallory than it is about MC/Amanda (or even MC/Mallory). Doesn't help that this revelation drops out of the blue with very little foreshadowing. As with the second case, I don't really mind this (as long as we aren't dealing with bruised feelings from the rejection going forward), but I can see why some people would object regardless of what word is used to describe it.
So that's my take, for whatever help it might be. Sorry for the excessive length, but as I said, NTR isn't a simple subject to discuss in any meaningful way.