As I understand it, some blokes, the cuckolds get off on their wife having sex with other men (bulls. Im sure, just like the particular kinks I have an interest in, it all gets very complicated after that. Hopefully a subject matter expert will pop up and fill you in.
a cuckold is any married man who's wife has sex with other men.
But the cuckold scene is a little more complicated... where most of the cucks like the idea of their wives being seduced by other men, or even going as far as being chastised about her affair / lovers etc.
The 'Bull' is her lover (usually characterised as being dominant / superior etc etc).
Predominantly a male fantasy, but some women enjoy it too.