It is out of politeness. Ladies do not "guffaw" out loud like men do. They are supposed to be delicate and demure.
In fact, in the West it used to be the same. Ladies would smile behind their fans.
Chinese traditional view thought the girl should be graceful and elegant. so people don't like the young girl mouth open so big like shark show their teeth, the true lady should not grin, they should be smile without show their teeth.(Chinese slang is 笑不露齿 )