-You are riding a beginner's unicycle with a saddle(seats) made of brick.
-You are not wearing padded cycling shorts.
-You do not know how to ride and accidentally rammed your crotch against the saddle.
Solution:
-Buy a better quality unicycle or just replace the saddle(*see below) with a better quality one.
-Wear a padded cycling shorts(**see below) and adjust your crotch to the front. Wear a tight cycling shorts so that the crotch won't slip back down. Since there is little padding on the front part of the cycling shorts, you might consider wearing the shorts front-side back. This offer maximum protection.
-Be careful when you are riding over obstacle, or hopping on the unicycle.
For the first 3 months when I was learning to ride, my crotch felt uncomfortable and I couldn't practice beyond 1 hour.
After I am able to ride, I bought cycling shorts and better quality unicycle. I could ride leisurely for longer hours (with breaks in between) and there is little trace of crotch discomfort.
I refuse to ride if I am not wearing a cycling shorts.
*Saddle with thicker, softer cushion is available at about $30 from:
EverythingBikez bicycle shop
**Buy at lower price from http://qoo10.sg/
(I've told you that online store sell things cheaper ... )
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Saddle modification ... I've never tried that ... but looks interesting though
http://www.youtube.com/watch?v=8cJQ4j5KOis

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Interesting post from https://www.facebook.com/groups/bccig/
??? If you want to NOT feel any discomfort down there, endure all the pain and ride on ... after a few years you will lose sensitivity to that area down there ...make sense ???
Or would you consider Trial Bike? Avoiding problematic crotch discomfort...
See action in Singapore http://www.youtube.com/watch?v=LsZ6496hH30