We are talking about NTR not being a cuckold. NTR is all about the feelings a person has knowing or believing someone they love is being taken by another. So even if the other person doesn't even know you exist, it is still NTR. In fact there is a whole sub-category of NTR for this. I think it is called "But I Loved Her First" or something.
With being a cuckold, if you use the literal definition then you still be one even if you two are not a couple. You just have to be such a weak loser that you willing support kids not your own.
Netorare is more often found in the form of the abbreviation NTR and is a controversial hentai branch built on cheating on the main character with someone else, as a hot bait to provoke jealousy in the viewer who wants to associate himself with one of the three participants in the plot: a playboy, a cuckold or a cheater.
In fact, the hentai artists revived the Italian commedia dell'arte with its cheerful Harlequin, crying Piero and nimble Columbine, who played with new colors in the light of the sexual revolution, bringing eros and masochism into the intrigue.
The world-old story of a love triangle added to the frivolity a taste of tragedy and suffering over long episodes instead of one-act vaudeville. This is the Japanese tradition of bringing everything to perfection.
The word cuckold means a husband deceived by his wife. This is an allusion to the mating habits of deer, which lose their mates when they are defeated by another male. This word often implies that the husband is deceived; that he does not know about his wife's infidelity and may not know until a child appears or grows up that is clearly not his (as in the case of cuckoo birds). It comes from the word cuck - a weak or servile person and from cuckoo - this refers to her habit of laying her eggs in the nests of other birds. The association is widespread in medieval folklore, literature, and iconography.
Note in the first case, a person becomes a cuckold when he already loses a partner, and in the second he does not even suspect that he has lost a partner. If we take the Japanese NTR, then more often it is based on the second option - someone else's child and a husband who suspects nothing
But in the West in 2000 they came up with - the cuckold became a fetish from the BDSM community in which the husband watches his wife being fucked and he likes it. In Japan, Netorase was quickly invented to define this.
In China, it's called "Putting on a green hat." It's strange that Fujino hasn't used it yet.
This is all I mean, basically that NTR is based on the attitude of 2 people into whom someone "third" climbs. And for NTR to work, the relationship between 2 people needs to be clearly highlighted for the players or the audience. If Asuna says or thinks (or Fujino writes in her status) that that's it, she no longer needs Kirito and the relationship is over and now they have nothing to do with each other, then I, as a spectator or player, lose all sense of feeling betrayal or jealousy. Kirito becomes a voyeur and a stalker. For NTR to work, Fujino must avoid ending the relationship permanently, even unilaterally. This is my IMHO and I think Fujino should insist to the end that Asuna "experience a bit of love or even without love, still have to stay with Kirito or marry Kirito"