"This is beautiful United! So Woke!" mocked user totalchaosx80.
"I identify as someone that should be flying first class, while only paying basic economy prices," wrote one twitter user.
Another said they identified as a suitcase, and insisted they flew for free.
Others purposefully misunderstood the MX to be an abbreviation of Mexican.
"I too identify as a Mexican" wrote @MartinBass200 – pointing out that '.mx' is the domain address for websites registered in the country.
For @legallycam225 said the airline had more important issues to fix before playing identity politics, telling United to "stop shrinking the seats and space to only fit 8 year olds."
Meanwhile some users welcomed the move.
"I love this invention," wrote @Lucas_Laura16. "Hope many companys [sic] follow."
Celebrity Drag Queen @Peppermint247 piled in to twitter storm and congratulated the airline saying it was now the turn of other airlines to follow.
United's chive consumer officer, Toby Enqvist said "United is determined to lead the industry in LGBT inclusivity, and we are so proud to be the first U.S. airline to offer these inclusive booking options for our customers.
"United is excited to share with our customers, whether they identify along the binary of male or female or not, that we are taking the steps to exhibit our care for them while also providing additional employee training to make us even more welcoming for all customers and employees."
While the move was mocked online, others defended it as brave and an important step to challenge social norms.
According to the United Nations statistics 1.7 per cent of people were designated as 'intersex' with neither male nor female traits.
According to the Daily Mail, 10 per cent of adults in the US identify as LBGT+, with even more among the younger millennial groups.
Many see large companies like airlines have an important role to play in adapting to new demographics.
Air New Zealand told Herald Travel that it was "currently exploring how we can introduce non-binary gender options across our various digital environments."
Currently New Zealand citizens already have the option to identify as indeterminate/unspecified on passports and travel documents, but airlines have to extend this title.