I'll frame this in the context of a heterosexual couple, because that seems to be the most common. "Cuckolding" is when a man and his partner derive sexual pleasure from the woman having sex with men who are not her partner.
Very often, her partner is "forced" to watch his wife or partner being unfaithful. The pleasure is derived from the humiliation the man feels at the idea that he isn't enough to satisfy her, so she must look elsewhere, too.
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Very often, her partner is "forced" to watch his wife or partner being unfaithful. The pleasure is derived from the humiliation the man feels at the idea that he isn't enough to satisfy her, so she must look elsewhere, too.