I recently watched a documentary about Princess Diana, and one of the commentators said that, historically, it was accepted that both English kings and their queen consorts could take a lover or lovers. In the case of Queen consorts, this was usually after they'd provided an heir to the Crown.
I am well aware of the stories of philandering English and British kings and princes, but to hear this was de facto with their consorts surprises me. Off the top of my head, I can only think of two consorts who we know cheated on their husbands: Queen Isabella, who ultimately deposed her husband, and Catherine Howard, who was executed. But if what this documentary said was true, then there must be some examples of consorts whose extramarital affairs ended more happily-- for everybody involved. Maybe.
Who were these queens? Who were their husbands? Did their husbands know their wives had lovers? What did their court think? Did any heirs face claims of illegitimacy because of their mother's affairs? If a woman was still fertile when these affairs began, was birth control effective enough back then to ensure she wouldn't become pregnant?
This question applies to queen consorts of Britain and England from the first English Queen consort all the way to Queen Elizabeth, the Queen Mother.
Hmm, it was actually very unusual for a British Queen Consort to take an extramarital lover, at least from the time of Henry VII onwards. (Earlier than that is outside my area of expertise). Two of Henry VIII's six wives were accused of adultery and subsequently executed: Anne Boleyn and Katherine Howard. The case against Anne Boleyn is a shaky one, and it is highly probable that she was innocent (as she maintained under trial).
After Henry VIII dies, there isn't another Queen Consort until the 17th century. Anne of Denmark, Henrietta Maria of France, and Catherine of Braganza (wives of James I/VI, Charles I and Charles II respectively) are not thought to have taken lovers. When it comes to James II, his first wife Anne Hyde was his lover before he was forced to marry her, but was never thought to have been unfaithful to him after their marriage. His second wife, Mary of Modena, was likewise faithful.
George I, who succeeded Queen Anne, believed that he did have an unfaithful wife, Sophia Dorothea of Celle. She certainly had a flirtation with a courtier, Count von Konigsmarck, although there is no evidence that the flirtation progressed to an affair. George divorced and banished Sophia Dorothea in 1694, imprisoning her in Ahlden Castle until her death in 1726. So, he acceded to the throne of Great Britain as a de facto single man. His children, including the future George II, never saw their mother again and never forgave him.
After that, you have Caroline of Ansbach and Charlotte of Mecklenburg, wives of George II and George III respectively, who were faithful and indeed regarded as patterns of female virtue. George III was the grandson of George II and Caroline of Ansbach; his father, Prince Frederick of Wales, had died before the king and so never came to the throne. Princess Augusta, Frederick's widow and George III's mother, was popularly supposed to have had an affair with the Earl of Bute, but it is unlikely that that actually took place.
George IV married Caroline of Brunswick in 1795, but the couple were estranged after the birth of their daughter, Princess Charlotte. Caroline travelled abroad for some years, and maintained a residence at Blackheath, on the outskirts of London. In 1806 she was subject to a formal investigation and prevented from seeing her daughter, due to rumours that she had taken numerous lovers. When George IV acceded to the throne in 1820, Caroline returned to London to claim her place as Queen, but was shut out of Westminster Abbey while the Coronation took place. Looking for a reason to divorce her, the King instructed Parliament to enquire into her affairs again. It was thought that she had had an affair with her Italian secretary, Bartolomeo Pergami. Despite this, she was popular with the public, and regarded as a victim of the King's selfishness and greed; dying in 1821 before a divorce could be granted. Unlike her predecessors, it is probable that Caroline did have some extramarital affairs, based upon the letters and reactions of those who knew her, although perhaps not as many as she was accused of.
After that, succeeding Queens Consort are not known to have been unfaithful. Adelaide of Saxe-Meiningen (wife of William IV) was a model of virtue, as were Alexandra of Denmark (wife of Edward VII) and Mary of Teck (wife of George V). The latter strongly disapproved of Wallis Simpson, for whom her son Edward VIII abdicated the throne in 1936. As did Elizabeth Bowes-Lyon, who married Edward's brother and successor George VI; i.e. the Queen Mother.
So you can see that, in fact, extramarital affairs by Queens Consort were exceptionally rare, and in those cases where they were suspected, the evidence was not conclusive. The reason for this is obvious: these women simply had too much to lose. In a society where female worth was predicated upon chastity and moral rectitude, and in a social structure (monarchy) which prized legitimacy and bloodline above all else, it wasn't worth the risk. Those women who did stray, or who were suspected of straying, were punished with ridicule, loss of status, the loss of their children, and imprisonment or even death. The majority, who remained faithful, were conscious of the power which their morality gave them, as national and religious figureheads, even when married to an unfaithful husband and king.
Related question: how about Queen Regents? Any recent scholarship on Elizabeth I?
Marguerite of France, the wife of Henry the Young (eldest surviving son of Henry II and Eleanor of Aquitaine), was accused of having an affair with the knight William Marshal by her husband, but Henry died of dysentery before he could annul the marriage.
Catherine of Valois took a lover and had a happier ending than Catherine Howard, but that's only because she had an affair after her husband, Henry V, had died. After becoming a widow, she became involved with Owen Tudor and bore him two sons before dying in childbirth. Henry VII was her grandson.
Marguerite of Anjou was accused of having affairs with Edmund Beaufort, 2nd Duke of Somerset and James Butler, 5th Earl of Ormond, both of who were always accused of having fathered Marguerite's only child, Edward of Westminster. This charge was bandied about almost exclusively by supporters of Richard, 3rd Duke of York, though, so it should be taken with a grain of salt.