I had a girlfriend when I was a teenager that had been quite promiscuous prior to meeting me, giving oral sex to various men. We dated for around five months and she would be very indiscreet, bragging about her past experiences... How she had sucked a guys cock at a train station, others she had left parties with and given oral sex to outside, let them go down on her etc etc. At first I was extremely jealous, angry, I hated the thought of my girlfriend being slutty and doing these things with other guys, especially given that she barely even knew most of them...
I had a porno at the time starring Jenna Jameson there was a scene of her giving a guy a blow job outside in a car park, she unzipped his jeans, without taking them off, and expertly sucked his cock. Jenna Jameson was tanned and with bleached blonde hair like my girlfriend of the time, seeing her suck cock from behind, watching her peroxide head bobbing up and down, it didn't take a huge leap of the imagination for me to imagine that this was my girlfriend. I think my mind made the connection involuntarily, and guess what? I was turned on imagining that imagining that I was watching my girlfriend sucking off the guy in the movie. I don't recall being shocked in anyway by this discovery, but I revisited the scene many times, imagining that it was her instead of Jenna Jameson.
Now this girlfriend also had incredible oral skills, probably better than any other girl I have been with since. She would wrap her hands around your ass and suck your cock so well you thought you were in heaven, she constantly varied her technique and would suck me for as long as I wanted, I'm sure at times she must have been sucking my cock for the best part of an hour. As she would do this I started thinking of how she had done this to guys she had only just met, imagining what it would have looked like, thinking how slutty she was. I started talking dirty to her more and more about these thoughts while she was sucking me; asking her why she was so good at it? That it must be because of all the strangers cocked she had sucked before. Asking if she liked sucking strangers cocks. I started calling her a slut, at first she was shocked at this, but then when I asked her about it after she said it was ok because "it made her heart beat faster".
I would ask her to recount every one of her previous exploits to me while she gave me hand-jobs, telling me about exactly what happened in detail, I no longer felt any jealousy about hearing her tales, just excitement. She sucked one guys cock in a train station why her friends waited around the corner telling her to hurry up, she left a party with another guy and sucked him while he sat on a wall, she took another back to her place and sucked him while he slapped her ass and called her a bitch.
We broke up before these fantasies progressed any further, if we had stayed together a little longer I'm sure I would have asked her if she was interested in sleeping with other guys, never the less, a seed had been planted...
