Well maybe it's got multiple uses, but the definition of cuck I've always seen used, context wise anyway, is referring to a guy who is letting some other guy have sex with a woman, usually implied to be their girlfriend/wife, and that they find that sexually arousing. And that the fact that they do this, makes them less of a man. Because no man would let someone else fuck their woman. Thus to me, it's still kink shaming. Maybe the guy is bi, or maybe he's just an extreme exhibitionist. Or maybe they all fuck each other silly after the other guy gets first dibs, I dunno. Don't really care, it's their sex life, and I'm not involved, so I'm not going to insult/shame this dude for liking it.
I've never seen it used outside of implied sexual connotations , usually questioning the guy's masculinity, as you say, but questioning specifically because of the sexual activities they are into.