Underwear, normal briefs that is, puts something tight around the groin. There is a large tendon that crosses the groin on the front inside edge of your thigh. Oh the pain and chafing in that spot if it's constricted while cycling. Oddly enough, I've occassionally heard it recommended that men who want to ride in underwear wear women's because it lacks seams in the center of the crotch, where men's underwear usually has a seam. Seams or no seams, it's the leg openings in a bad place that I could never stand. Maybe seamless boxers would be more tolerable, but commando is the best way to go for most of us.