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.She's not his LI though
as Snugglepuff has explained a few posts ago
and if someone is unbothered it would be an open relationship rather than cuckoldry
and - of course - there is no cuckoldry (or open relationship) tag
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.