'Advice' given when a woman will inevitably be taking part in unwanted but mandatory/inevitable sex, usually with a partner but sometimes attributed to rape.
Apparently the phrase began in Edwardian England, when mothers would give it to their daughters on their wedding nights- this would be then first girls had heard about sex. Basically, think of the good the children could do the
country (could become soldiers, doctors, etc).
No matter geographical location, 'England' should
always be used- don't go about saying 'lie back and think of 'Murica' or 'lie back and think of
India'. People will think you're a right
ninny.