Its a weird twisted thing following from the trend that started in th 60's. Women started wearing pants, and Fundamentalists took Deuteronomy 22:5 "The woman shall not wear that which pertaineth unto a man, neither shall a man put on a woman's garment; for all that do so are abomination unto the LORD thy God." and their application is that because traditionally, culturally in America, men wear pants and women wear dresses, a woman weaing pants was abomination. And even though it became culturally acceptable for women to wear pants, it was ensconced in Fundamentalism as an unchanging standard. I even heard Jack Hyles say that Jesus wore pants. All the Jews wore pants. They did not wear robes.
The longer it went on, the more they tried to define it, until the definition, in Bob Gray's case, became, anything that had legs in it, even pajama pants or shorts, was sinful.