well it did say 'extreme' biking.
But you raise a good point (pun fully intended!). The whole boxer/brief thing is another issue entirely.
Also, if some studies are to be believed, if you drink your water from Thames Water will give you low count/motility.

Ah! The delights of surveys and studies. Lets compare 50 guys in briefs riding bikes with 50 couch potatoes in boxers who live in London