In my case, it was my now ex. We had been dating since I was 16. He's the only guy I have ever slept with. (Poor dad still thinks I'm a virgin.) One night I went over to his house to meet him after he got off work. I was honestly hoping we'd had the night there at his place and wanted to spend it in bed. When we got there he said it was a long night at work and he had to tell me something. He had made some stupid bet on some game (NFL playoffs I think) and lost. I asked how much he lost, thinking it was just gonna be me paying for dates for a long time. He said well the bet was $500 for him if he won and if he lost (which he did) he would let the guy he made the bet with have sex with me.
I couldn't believe him, I asked what the hell he was thinking. He said, "I'm sorry but you have to let him, I made him a deal." I slapped the hell outa my (now ex) boyfriend. As I got in my car another car pulled into the driveway. I rolled down my window, "sorry he doesn't have a girlfriend, you all will have to work out the $500 some other way."
The other guy wasn't bad looking, it was just the idea my guy thought he could just make me have sex with any random guy he made a be with on something that I will not tolerate. Almost two years of a relationship, gone in a second. Part of me misses the good times with my ex, but I will never have a relationship with someone that thought that was ok.