Just imagine how inflexible a pair of theoretical 1000D boxers would be. I think I'd rather stick to my chainmail underpants, at that rate.

Wending our way back to reality, I do have a pair of cycling underpants (like, pedal bikes) with baboon butt padding in them. I bought them in the hopes that it'd make the 2x4 my Orion RXB250L calls a "seat" vaguely tolerable any time you have to be astride it for more than 15 minutes. They almost work.