A United Airlines gate agent barred two girls from boarding a flight Sunday morning because the girls were wearing leggings.
Another girl who was wearing gray leggings had to change before she was allowed to board the flight from Denver to Minneapolis, a witness said.
"She's forcing them to change or put dresses on over leggings or they can't board," Shannon Watts, who was at a gate at Denver International Airport, said on Twitter. "Since when does @united police women's clothing?"
United, responding to tweets about the incident tweeted that "United shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage." And added, " This is left to the discretion of the agents."
The airline's passenger contract says for the safety of all passengers and crew members, the airline can refuse to let a passenger on board if the passengers is "barefoot or not properly clothed."
The airlines, however, does not define "properly clothed."
Social media exploded Sunday with users called the incident horrendous, outrageous and a nonsense. Several users asked why leggings aren't proper clothing and promised to take their business -- and leggings -- to another airline. Many women wear leggings, yoga and athletic gear for comfort while traveling.
Watts was traveling on vacation to Mexico when she heard a family next to her panic. Then she heard the female agent explain to the young travelers that they couldn't get on the plane wearing Spandex.
"I don't make the rules, I just enforce them," the agent told the passengers, Watts said.
The young girl, who was wearing gray leggings, not too tight, said Watts, pulled a dress from her backpack and put it over her leggings and was allowed to board, Watts said.
But two other girls were turned away and not allowed to board because they didn't have any other clothing, she said.
"I have five kids: four of them are women. They wear yoga pants all of the time when flying," said Watts in an email from aboard her flight. "I think this policy is arbitrary and sexist. It singles out women for their clothing and sexualises little girls."