The cuckold fetish is an atavism from "the sperm competition". It's a natural ******* instinct of survival. The vast majority of ******* species fuck due the reproductive instinct. Females choose male due their strength, health, dominant carácter and more. If one male is looking for females to breed, probably this male must fight for her. If female had been fucked before, the new male will try to fuck them even better.
In humans occurs something like that. If one husband has been cuckolded by his wife and he knows, probably he is going to demonstrate his wife that he's even better than the other man: more virile, stronger, more handsome.
But his ******* instinct has a weird way to cerebrally activate this mechanism: making him feel more excited. This is the sperm competition.
Some men have that basic instinct more developed and turned a real fetish. In this way it's not weird that a lot of men turn on just imaginning their wives fucking with another stronger and more virile men th