If planning a wedding weren’t stressful enough, one Hong Kong bride-to-be now has to worry about the possibility of 1,000 angry gate-crashers.

The crowd has risen up in response to her widely republished Facebook posting earlier this month: “I’m not opening a charity….If you really only want to give me a HK$500 [US$65] cash gift, then don’t bother coming to my wedding.”

Social-media...