I don't wear undies for any activity involving repetitive motion "down there"...not running, not biking, not nookie...

Seriously, get over the commando-phobia. You will be SO much happier. I ran for a year or two and tried every "sport" panty out there. They all gave me bloody welts. I cannot imaging biking with any sort of seams near my privates, bottom, or thighs. OW!