"Cuck" has lost most of its meaning in modern discourse, and most of the times its meaning is "is with a woman who wasn't a virgin" or even just "got disrespected by a woman."

Its the "Right's" version of things like "incel," where its use is meant to be emotionally charged rather than accurate. Because no one wants to be a "cuck" or even "defend cucking" so you can just use it liberally and everyone continues to go along with it.