You will find that the term “naked marriage” is a lot older than 18 months if we accept the 'peopledaily' explanation.

“The "naked marriage" phenomenon, which was coined in 2008 by Internet users, has drawn much discussion after a popular TV series called "Naked Marriage[Wedding] Age" struck a chord with the Chinese youth, especially those born in the 1980s.”

http://english.peopledaily.com.cn/90882/7562222.html

"Naked Wedding Age" was a Chinese drama that was first shown in May 2011.

Not to be confused with the other use when young married couples pose naked or semi-naked for professional and personal photographs - a practice that has been slammed by the 'moralists'.