This was a small but successful company; my boss was "the HR". I was like 26 years old at the time and obviously physically healthy.
I wasn't fired for wearing diapers the next day, I was fired for "being late", you know? But it was absolutely for wearing diapers; he told me to "wear my big boy pants to work" when we got to the top of the staircase and the next day I was fired for some really minor issue - especially considering the unpredictable commute driving 1.5 hours to work.
Anyways, a lawsuit would not have gotten me anywhere, just led to more public humiliation. It's not like I had a medical reason to wear diapers.
So - there's a lesson here. Sometimes it's best to keep work and pleasure separate.