Charles the Adulterous Prince Regent.
“Indeed, my Lord, it is an absolute and most excellent horse.” —Henry V
In the old days it was pretty much taken for granted that kings had mistresses. Henry VIII had a problem only because he wanted a son and the son could only inherit the throne if born to his lawful wife. Charles II was notorious for having numerous mistresses...Princess Diana was descended from one of his illegitimate children. James II was driven out not for having mistresses but for other reasons.