I would start by asking her guys she thought were hot and to fantasize about them while fucking me. At first it was just celeberities etc. Eventually I asked her about if she liked anyone we both knew. Finally, she began fantasizing about a friend (or kinda friend, not one anymore) of ours. This began to increase and become more detailed. We were drunk at a party and she accidentally kinda mentioned it to the guy and he then began to persue her. Eventually, she took the fantasy and fucked him. He was the second guy she had ever fucked since her and i started dating before college. 

This eventually led to her fucking other men as well. I wanted it to stop as I wasnt able to handle the fantasy in real life but kept jerking to her.. Then after orgasim asked her to stop then got horny again and pushed it and so on. Basically she didnt stop... I think once she realized how hot she really was due to all the attention she was getting and me telling her about what my friends said and other guys etc, it increased this. 

When we were younger in hs and freshman year in college... her confidence wasnt as high. Which is typical. So as it grew, so did the fantasy and her desire to act on other guys hitting on her etc. 

this ultimatley led to her dumping me.  

 I still love her and jerk off to her often.