Baffled users online also pointed out that nobody is wearing face masks in the video, suggesting it was filmed before the coronavirus outbreak which brought China to a standstill from late January.
“This woman is so brave, I don’t understand, [she’s] just doing it in broad daylight,” read one Weibo post that gained more than 8,000 likes.
“There are so many people around, I just don’t understand,” another wrote.
It is not the first explicit video to cause a stir on China’s tightly controlled social media platforms.
A Beijing branch of the Japanese clothing chain Uniqlo became infamous in 2015, after a clip of a couple having sex in one of its changing rooms went viral.
Police arrested five people, including the young couple in the video, over the matter, while Uniqlo firmly denied that it was a publicity stunt.
The clip “severely violated socialist core values,” the Chinese Cyberspace Administration said at the time.
Chinese citizens can face up to ten days’ administrative detention for deliberate public nudity, while those who upload and disseminate obscene content online face up to 15 days’ detention and a maximum fine of 3,000 yuan.