'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.