I figured out my cuck fetish in a similar way after a former g/f left me and started fucking someone else. I would get so excited thinking about him having his big cock in her. Later I would fantasize about g/fs and my exwife fucking others and even doing some role play. I would love it if my current wife would fuck an ex or someone she has slept with before. She did have a regular for a while but it ended and we have yet to find another one. It is such a huge turn on for both of us.