Welcome to the world of Tinia Cruris (jock/dhobies itch) or Ring Worm.
Canestan or Daktacort. As stated if you have family keep your own towel.
Change your riding shorts regularly. Every ride or day if you commute. After long rides have a shower, bath, wash your privates and try wearing loose trousers for a time with no pants.
Once cleared try applying athletes foot powder in your pants if you commute and can’t “clean up’ easily.
The problem is man made fabrics and wool require cool washes that won’t kill off the fungus or spores. Airing your shorts inside out on sunny days will help as the fungus don’t thrive in UV, which is why our sock shrouded toes and the groin is just a perfect home.
Note of caution. I am allergic to Canestan. The treatment made things VERY bad for me. If you don’t improve in a week or things get worse stop using whatever and get to the doctors. You’ll be amazed at how bad things can get. (for example you may imagine a scenario where you have to wear sanitary towels in your pants to absorb the weeping puss). I kid you not. Almost 20 years later and not been right since.