But that's not how NTS works either. NTS is a genre in which the fact that a husband or wife has a fetish in a loving couple appears (We have a progressive society). He wants to see his wife with another man, for example. "Darling, I have a wish, I want our neighbor's son to fuck you" "Yes, darling, I will do anything for you." NTS without feelings in a couple is voyeurism. Kirito just likes to watch people who don't really care about Kirito.
And if you look at the terms, then Klein in the latest update is more of a voyeur than cuckold. After all, Alice doesn't give a damn about Klein either.