One should never wear their underwear under cycling shorts. Wearing it will cause friction and ultimately lead to chaffing. This can will create a very uncomfortable ride. Cycling shorts work best when they're next to the skin. Underwear under bike shorts are hot, not sanitary, create unsightly "panty lines."
WE STRONGLY RECOMMEND NOT RIDING IN UNDERWEAR and getting a good pair of shorts. Luckily, Bicyclinghub.com can help with this!