With me it was my exwife that got me into it. We had talked about how I felt watching other men entering her and I told her it was a turn on as another cock, preferably a thick one, pushed into her tight hole. I relished the sound it made as it went in with an almost popping sound then the full length disappearing from view. Then the balls slapping on her thighs made me want to suck on them as they started building up a rhythm. That's when she hatched a plan to trap me into it. Its a long story, but in the end I was fucked and sucked, very roughly, by 5 other guys.
Afterwards I was sore, but it felt amazing. Since then I have looked at other men in a whole new light and when me and the exwife split and I met another woman, after we started swinging together, I confessed my attraction to other cocks and surprisingly she encouraged me to carry on while she was with me so she could watch.
I cannot bring myself to kiss another guy yet I love to suck cock and balls, and yes, I even got to play bottom to some of her bulls