You're right, but the problem is his mindset. Most of these guys that like to join in when their wives are getting fucked don't even consider themselves cucks, they tend not to like the humiliation stuff of being caged, eating creampies, etc. I've met some like them. These guys often don't see themselves in a submissive position, they tend to see themselves as the dominant one, the one who allows his wife to get fucked because she's his bitch and he owns her, stuff like that. So even if he acts as a traditional/stereotype cuck a few times he'll probably be conflicted. They probably weren't ready to take this step and fucked up their relationship. Just a supposition of mine