As has been the case for nearly all of human history, 18th century marriage was mostly a financial transaction. Even in the lower classes, people were concerned about bringing together two people who could make a living and bear children. Brides were often pregnant, because fertility was such an important requirement. The poorest of the poor often didn’t marry; they had no property, money or prospects, and marriage for love was very rare. The upper classes certainly married for money, and also for influence. Powerful families gave a lot of thought to the compatibility of their properties and their politics, and much less to that of the offspring involved.
A Most Unusual Situation is loosely based on a true story of 18th century marriage, that of Charles Lennox, the Earl of March, son of the Duke of Richmond, to Lady Sarah Cadogan, daughter of the Earl of Cadogan. Cadogan famously lost thousands of pounds to the Duke of Richmond in a game of cards, and literally married his 13 year old daughter to the Duke’s 16 year old son a day later at the The Hague (in the Netherlands) to pay off the debt. She was terrified, and he was infuriated; he displayed it by leaving on the Grand Tour of Europe the next day, not to return for four years. When the Earl of March and his wife were reunited in London, however, he discovered that a great deal had changed about his wife.
This story fictionalizes their reunion, and the nature of their love story. However, their real life marriage is generally believed to have been very successful by 18th century terms, if for no other reason than that it produced seven children who grew to adulthood, including the all important heir. Modern readers will be happy to know that the marriage itself was happy and that the Duke and Duchess were also very fond of each other. The story of three of their daughters is the subject of an excellent biography written by Stella Tillyard, The Aristocrats.
Portrait of The Duke and the Duchess of Richmond, by Jonathan Richardson. This portrait was made at the request of the Duke's grandmother Louise de Kérouaille, who wanted a portrait of her grandson and his wife (wikepedia.org).