In "The Lady or the Tiger?" by Frank R. Stockton, if the princess had married her lover, what would this be an example of?

When a man or woman of royal blood falls in love with and marries a commoner, this is known as a morganatic alliance or marriage.

The term "morganatic" refers to the German "morgengabe," which is the gift bequeathed to the bride by her royal groom, the morning after the consummation of the marriage. Historically, in Germany, the commoner spouse and any resulting children from the union could not inherit the titles nor the privileges that came with the royal husband's position.

In ancient times (as in the story), morganatic alliances (whether they resulted in marriage or not) were frowned upon. The consensus was that royalty were expected to behave according to prescribed social norms. Therefore, for the purposes of preserving the royal bloodlines to the throne, only marriage or sexual unions with other royals was considered acceptable.