Two teenage girls were told that leggings weren't allowed on a flight from Denver to Minneapolis, a United spokesperson said.
"They were flying under the employee pass travel privilege," Johnathan Guerin said.
Employee passes carry a different set of rules, since those travelers are flying as representatives of the company, Guerin explained. The dress code bars pass travelers from wearing spandex or Lycra pants such as leggings.
The dress code does not apply to passengers who are not flying with the employee privilege. The girls changed and were allowed to board.
Activist Shannon Watts of Denver tweeted that she witnessed Sunday's events and questioned United's decision to police women's clothing.
Watts said the girl's father was allowed to board while wearing shorts and called the airline's policy sexist.
Twitter users were still outraged.
Some users said even if the girls were flying with an employee pass, the boarding agent should've ignored the rules.
The Associated Press contributed to this report.