It’s a little strange to think of the kings of France and England having official mistresses, when considering how Catholic they were, and how much they cared about female virginity and fidelity. But throughout the 16-1700s, maîtresse-en-titre or official mistress, was a position in the French court that came with apartments and a small salary.
These women were not the lower class prostitutes favored by other men, but noble women in the Courts of kings. Sometimes they were married and sometimes they were single. The position came with certain powers and privileges, and a few mistresses were able to secure land and titles for themselves and their royal bastards.
There were a few factors at working supporting these public displays of infidelity. One was the widely held belief that sexual intercourse during pregnancy could induce miscarriage, so sex with a pregnant queen was to be avoided. Another factor was the nature of royal marriages: they were about treaties and bearing royal heirs, not love or attraction.
Only kings and crown princes got this marital “out” for pursuing love and sex, never queens and princesses. A married queen’s first responsibility was to her king. Her job was to produce legitimate heirs to the throne. If she had sex with other men, then the bastard child of a foreign queen might take the throne.
Kings got to stay in and rule their own country, while most queens lived isolating lives in another country with another language following their marriage. If anyone needed some love and comfort, I have to imagine it was them. But that was strictly forbidden, on pain of death. That doesn’t mean it never happened, but that it was never an official court appointment.
Most of these kings had more than one mistress over the course of their reign. While maîtresse-en-titre was a somewhat respected position, former mistress was not. A former mistress was a woman without her maidenhead (virginity) who was no longer in the king’s favor. There was very little left to protect such a woman from ruin. If she was married during the royal affair, she might be able to return to her husband. If she was single, she was unlikely to ever wed.
I think the ultimate reason so many kings had mistresses was because they could. Who can say no to the king? Not a lord whose wife the king covets, not the lady herself. The power imbalance was enormous, and since the Vatican was not opposed, no one had the power to tell kings they shouldn’t.
Infidelity among political men is still common, though no longer officially tolerated. A Senator can lose his seat now for an extramarital affair, yet they are still common. So long as the majority of world leaders are men, and “owning” or bedding many women is believed to be a sign of power and virility, I think men will have mistresses, official or otherwise.