lie back and think of England

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• verb idiomatic In imperative form, advice supposedly formerly given to women before sexual intercourse with their husbands. (often associated with the Victorians, though not attested until later). "think of England" refers to the importance of children.
• verb idiomatic, humorous, figuratively Used to preface any unpleasant but inevitable experience.