Bluntly speaking, I consider "open relationships" as what you get when you're either a cuckold in denial, you don't enjoy fucking your spouse anymore but don't want a divorce, or you're not emotionally invested in the other person anymore (or never were) and just want a fuck buddy without any strings attached.
There's nothing wrong with any of those scenarios. But, aside from being a cuckold, it implies a degree of emotional ambivalence for the other party such that, quite literally, you don't care if they make love with other people. Which is an entirely subjective nuance that depends on the player's perspective. For those who want romance with the characters it translates as either cuckoldry or NTR.
tl;dr Your opinion that a situation constitutes an "open relationship" is exactly as valid as the opinion that it's cuckoldry or NTR.
Regarding the term "open relationship"
I certainly can't find what you consider the term to mean in any dictionary
It is clearly not synonymous with cuckoldry or any form of NTR in any dictionary I can find
It is a different term used for very different circumstances
There may be games where it is difficult to judge the nuances - and therefore the appropriate term - this game is not one of them
It is very clearly set in an environment where the MC (Krowly) and all the NPCs are comfortable with shared sex
Krowly happily has sex with many different NPCs of differing genders
Krowly happily partakes in threesomes and foursomes
Krowly happily voyeurs on NPCs having sex
Krowly happily voyeurs in the company of other NPC
Krowly invariably faps while watching others have sex
Krowly discusses with numerous NPCs enjoying having sex with other NPCs
Most of the major NPCs - if not all - have sex with at least one other NPC
Some NPCs strongly encourage Krowly to enjoy sex with others
All of which strongly indicates a glut of happily consensual open relationships
If these scenarios were to seriously upset anyone it would probably be sensible for them to look for other games