When people use the term cuck they dont USUALLY mean it for its literal meaning, they use it as an insult to NTR players. If you like any form of NTR you are a "cuck", "beta", etc to these types of people.
This can be confusing because cuck can be used to accurately describe a form of NTR which is Netorase, in a Netorase you watch the girl get fucked and you like it, that is closest to definitive actual cuck.
But again, people rarely mean it that way, they just like calling NTR players cucks as an insult.