As simple as it sounds. Sometimes our kinks are mysterious, but sometimes you can look back and see a clear origin for them, whether that be a traumatic response or a childhood hobby or something.
For me? While figuring out my bisexuality, i also played a lot of fighting games.
Seeing two female characters throw each other in Soul Calibur or Dead or Alive was one of the earliest examples of female physical contact that i got that my parents didn’t immediately try to ban so like… it stuck with me and now as an adult i get turned on by certain kinds of fights or martial arts gear.
Looking through a mail order catalog and going into the ladies swimwear section awoke something in me. Also the first time I tried wearing a speedo, it gave me an instant erection.