Wiktionary actually places the "unaware" or "unaccepting" aspect as the primary definition:
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This makes sense, because "cuckold" as a term derived from the cuckold bird placing its offspring in unsuspecting bird’s nests. And this ignorance is a prevalent theme among many NTR doujin. In the comments section of many NTR doujins, users still refer to the unknowing husband as a "cuck/cuckold." It’s a fitting term for someone who raises, or will raise, someone else’s kids, heh.
I get that cuck/cuckold has a negative perception (and many don't like being labeled that), but the more specific term "wittold," is super archaic that no one uses. So "cuck/cuckold" is used in popular vernacular to cover both cases now.
There's also a sexist undercurrent behind it too. That a man who can’t please his wife (and stop her from cheating) is no man at all. So even if he doesn't know about the cheating, it’s still somehow his fault for being a beta that can't please a woman.
Some people will exclusvely use cuck/cuckold to imply a man
knowing and enjoying infidelity, but not everyone does.
For a lot of people, if they see a man cheated on in a doujin, he’s a cuck/cuckold (like every housewife doujin lmao). Regardless of whether he finds out in the end.
I don’t personally mind which terminology people use for it, it’s pretty easy to gather from context, but regular Joes do not make the distinction between cuckold/wittold, so everything is cuckold to them (and they are actually correct in this usage, like calling a chihuahua a dog).