Louis I, Grand Duke of Hesse
In 1776, he became engaged to Sophie Dorothea of Württemberg, eldest daughter of Frederick II Eugene, Duke of Württemberg. The engagement was broken off so that Sophia Dorothea could marry Louis's recently widowed brother-in-law Tsarevich Paul Petrovich, son and heir of Catherine II "the Great", Empress of Russia. He received a monetary compensation when the engagement was broken.
Morganatic marriage
Perhaps the most famous example in modern times was the 1900 marriage of the heir to the throne of Austria-Hungary, Archduke Franz Ferdinand, and Bohemian aristocrat Countess Sophie Chotek von Chotkowa. The marriage was initially resisted by Emperor Franz Joseph I, but after pressure from family members and other European rulers, he relented in 1899 (but did not attend the wedding himself). The bride was made Princess (later Duchess) of Hohenberg, their children took their mother's new name and rank, but were excluded from the imperial succession. The Sarajevo assassination in 1914, killing both the Archduke Franz Ferdinand and his wife Sophie, triggered the First World War.