sharing his gf with a big dick

In fetish usage a cuckold or wife watching is complicit in their partner's sexual "infidelity"; the wife who enjoys cuckolding her husband is called a cuckoldress if the man is more submissive. If a couple can keep the fantasy in the bedroom, or come to an agreement where being cuckolded in reality does not damage the relationship, they may try it out in reality. However, the primary proponent of the fantasy is almost always the one being humiliated, or the "cuckold": the cuckold convinces his lover to participate in the fantasy for them, though other "cuckolds" may prefer their lover to initiate the situation instead. The fetish fantasy does not work at all if the cuckold is being humiliated against their will. Psychology regards cuckold fetishism as a variant of masochism, the cuckold deriving pleasure from being humiliated. In Freudian analysis, cuckold fetishism is the eroticization of the fears of infidelity and of failure in the man's competition for procreation and the affection of females. In his book Masochism and the Self, psychologist Roy Baumeister advanced a Self Theory analysis that cuckolding (or specifically, all masochism) was a form of escaping from self-awareness, at times when self-awareness becomes burdensome, such as with perceived inadequacy. According to this theory, the physical or mental pain from masochism brings attention away from the self, which would be desirable in times of "guilt, anxiety, or insecurity", or at other times when self-awareness is unpleasant. Source : Wikipedia