I should imagine that silk underwear is usually saved by us ladies for "special nights," it isn't something we would wish to wear all the time, believe me it isn't as comfortable as cotton underwear and probably not as comfortable as men's boxer shorts. Women's butt's often munch half their underwear as soon as they sit down no matter how tiny their ass is.
Silk feels nice against the skin, but is no good in hot weather, and doesn't breath as well as cotton, so men stick to your own pants and boxers :D