Sex life

Hi We are a couple and i have a fantasy about my girlfriend being with another man she once talk to about while having sex and now i can't stop thinking about why am i so turned on about this fantasy

Nothing wrong with fantasy, but reality might get in the way. Do you want to play the role of the cuckhold in this relationship? (look it up)

I've seen cuckold in porn and it turns me on just confused why it does so much