I have fantasies about my Fiancé’s co-worker I met once!

I (34F) have been with my fiancé (37M) for 7 years. Recently we lived interstate (relocated for his work temporarily) and at the work Christmas party one of his coworkers sparked my interest.
In general I don’t find many men attractive. I know when someone is good looking but finding them attractive happens so rarely for me.
My attraction towards this stranger was instant. He was with his partner that night too.
We barely spoke but I haven’t been able to shake the fantasy of him.
My relationship with my fiancé is fine. I wouldn’t say perfect but we are happy most of the time.
I don’t know why I can’t get this guy out of my head.
I don’t feel bad about it and part of me wishes I could fulfil my fantasies with him. He lives on the other side of the country now and I wonder to myself if given the opportunity would I?