The mal mariee
(French for "badly married") is a young woman unhappily married to a much older man. She is almost always innocent and beautiful, and was likely married against her will. He is probably ugly, probably cruel, and definitely obsessively jealous; he often keeps her imprisoned (most iconically, in a tower) to avoid her interacting with other men.
This never works. The more he tries to keep her for his own, the less she wants to do with him - and sooner or later some handsome young knight, poet, or fairy will find a way to romance her.
of Sympathetic Adulterer
very common in medieval literature, particularly in troubadour and trouvere poetry and the lais of Marie de France. A Forgotten Trope
now, though it may get nods in stories with medieval or medieval-inspired settings.