I think it had to do with sexualized fear/jealousy. My best friend in high school was very well hung and had quite the reputation. He knew I really liked this girl I was starting to date and he started to really flirt with her and she was loving the attention. I told him to knock it the fuck off and he did. My girlfriend was a bit bummed he stopped coming around but I had become really jealous. His cock was more than twice the size of mine and he had a confidence and swagger that made girls lust for him.
I also remember watching the playboy channel when I was in high school and saw some movies where: 1) a reluctant wife gets pressured into swinging by her husband even though she really didn’t want to. The man from the couple they swapped with had a much bigger cock and the guys reluctant wife was totally going off on his big cock and her husband had MASSIVE regret watching her have orgasm after orgasm with her bigger dicked partner. Was so incredibly hot. 2) A young virginal princess was being transported by carriage to her fiancé so they could be married. A group of pirates/marauders overtook their travel party killing all of her protectors. The gang then took their turns fucking her. She resisted early on but as the days wore on she started to really get off on them fucking her roughly. Again, super hot.
When I got to college I was still a very jealous guy. I met my college girlfriend and we fucked all the time. Later I started to have fantasies about my cute Catholic college girlfriend fucking a law school friend I met who was super confident, a quiet swagger, even a bit cocky. Looks wise he was nothing special but he had a sexual magnetism that was undeniable. After my girlfriend met him, I asked what she thought of him. She blushed a bit and said, “It’s strange, he’s not good looking…… but…. He is so damn sexy… wow.” I told her I understood what she meant.
The first time I introduced him into our sex talk while we were fucking around, I started by saying, “what if you were alone at my apartment and Rick was there taking a shower, you are drying your hair in the other bathroom and he walks in naked looking for a towel. You look down and he has a very long, thick cock, much bigger than mine. And he tells you that he didn’t really need a towel, but he wanted to fuck you? Would you fuck him? She shyly replied, “Well maybe just that one time, just to see what it would be like. But that would be it.” Then I said, “Yeah but what if he fucked you so good, you had multiple orgasms, the most intense ones ever, just blew your mind?” Then she said with a smile, seeing that I was really turned on, “Well in that case, I’m sorry baby but I guess I would just have to keep letting him fuck me with his massive, amazing cock, I mean what could I do? He fucks me so good, and so deep, how could I possibly resist.” We fucked like dogs in heat for the next hour.