That's already answered by many of the user's in last page.
Did I say Alison's case is NTR? nope! I DIDN't EVEN MENTIONED
NTR.
Anyway It all depends on whether how Ian and Alison feel about each other If in one playthrough both are emotionally involved(Alison is always involved so in this case Ian would be the factor to consider) than yes! that would fall into NTR category and NTR covers
That's why girl's in a Harem are called
CUCKQUEENS!
Than it mean's you have a cheating kink!

(Either doing cheating or being cheated on)(That's why I play only MMC games and usually avoid FMC ones until FMC and MMC both are affecting each other's choices/consequences and NTR is avoidable

) now this kink may be limited to VN's or fictional material only or you may subconsciously desire it. Now keep in mind here I referred as you not to point out you(mommysboiii) but all those who have this scenario going on!
Your opinion's are subjective and mine too, we don't have to be on same page.
Now consider this scenario:
They are in relationship
- John and Mariah
- Mark and Daisy
- Liam and Kate
John and
Mariah love eachother but
John with
Daisy whose boyfriend is
Mark and
Mariah with
Liam whose girlfriend is
Kate are cheating on eachother. Now remember Both
John and
Mariah are
cheating on eachother. In
John's case
Liam is
cucking John and
Mariah is
cucking Kate. On the other hand
John is
cucking Mark and
Daisy is
cucking Mariah. In this scenario both
Daisy and
Liam are also
cheating on their partners but
Daisy and
Liam are not being
cheated on they are doing
cheating. From
Daisy and
Liam's perspective it would be
netori.
If they all don't give shit about eachother(means not emotionally, mentally are involved with eachother) are are not in any sort of relationship/commitment than no one is getting cheating on or be cucked.
It's all about perspective and how emotionally invested you are in that person.