Why has badminton become a code for sex in Hong Kong?

It may be an innocent enough racquet sport, but Hong Kong's Education Bureau has unintentionally given badminton a whole new meaning. 

In teaching materials it released last week, a module titled adolescents and intimate relationships for Secondary Year 3, suggested that teenagers who wanted to have sex with each other could "go out to play badminton together" instead.

The materials also include a form called "My Commitment" aimed at getting "young lovers" to attest that they would exercise "self-discipline, self-control, and resistance to pornography".