the saddle on the MTB is um, old. The bike is a Kuwahara Savage. I also have a low-end unisex hybrid with a different, cheap saddle that has never caused me a problem.
I think my wearing of just my cheap tights and then, not being able to change out of them immediately as my son required Boobies Right Now etc probably led to this issue...I hope.
The weather is about to turn to poop here and I'll be off the bike for a couple of days due to the owie and the weather so time to heal.