Boy Left Girl Right

The other day I saw a few students taking a photo together and as they were arranging themselves the girl holding the camera shouted:

"Nán zuǒ nǚ yòu" 男左女右 [boy left girl right]

This seems to be a common/traditional way of arranging a boy and a girl for a picture or on stage hosting an event, etc. Also, while it's not 100% consistent, I started looking at xǐ shǒu jiān 洗手间 arrangement and noticed that most follow the same pattern.I asked my students what the origin of this little phrase is. One student said that in Traditional Chinese Medicine, the doctor would take the pulse (bǎ mài 把脉, for you Word Hogs) of men using the patient's left wrist and use the right wrist of a woman patient.Also, apparently a married man puts his wedding ring (if he has one) on his left hand's ring finger and a woman on her right hand's ring finger.Has anyone else:1) Heard this phrase?2) Noticed it's usage beyond photos, toilets, and wedding rings?3) Learned the origin of where this came from?If so, please enlighten us (or at least me).(from left to right) CCTV's Zhū Jūn 朱军 and Dǒng Qīng 董卿