A United Airlines agent in Denver on Sunday morning barred two teenage girls from boarding a flight to Minnesota because they were wearing leggings. A third passenger, estimated to be about 10 years old according to observers, was able to satisfy the agent by donning a dress over her leggings.

The no-leggings rule applies only to employees or to travelers using a United perk — often called a buddy pass — that allows friends and family of employees to fly for free or at a discount. A spokesperson explained that those travelers are representatives of United when they fly, so they're expected to follow the company's dress code. Even teenagers and little girls in leggings, apparently. Leggings that you'll find females wearing ... everywhere.

"The passengers this morning were United pass riders and not in compliance with our dress code for company benefit travel," United said. "We regularly remind our employees that when they place a family member or friend on a flight for free as a standby passenger, they need to follow our dress code."

Most mornings, LeggingGate probably would have gone unnoticed. But as circumstances would have it, a woman at a neighboring gate watched the scene unfold and began to tweet about it. As circumstances would have it, she's an activist with more than 34,000 followers on Twitter. She's also a mother of leggings-wearing daughters and found the company's policy sexist — and pointed that out on social media.

The conversation quickly became: How many men get snagged at the airport by clothing restrictions? Does United's policy prohibit young girls from wearing casual clothing but not prohibit boys from doing so? Women but not men? Leggings are banned but athletic pants are not? Why?

United staunchly defended the policy, answering critics with curt "follow the rules" tweets. The statement posted to its website restated the policy but added: "To our regular customers, your leggings are welcome."

Not everyone was sympathetic to the travelers. "Get over it," some posters on Twitter responded. If you're flying on United's dime, you don't get to complain about the rules. We get that.

But the optics don't work in the Chicago-based airline's favor. What's the point of zero tolerance here? The agent singled out three girls because their clothing was deemed inappropriate. They were dressed like typical American females, indistinguishable from full-fare passengers whose attire United acknowledges it finds perfectly acceptable. Many of the people who witnessed the scene — or read about it — were likely wearing leggings at the time.

And a public relations crisis took flight.

FROM AN EDITORIAL IN THE CHICAGO TRIBUNE