hi! there's lots of room for more contributions on this, but do check out this thread on Elizabeth I of England ~

Why could Elizabeth I not marry the Earls of Leicester or Essex even though her father had married relatively unimportant nobility?

Well Isabella and Ferdinand were Co-Rulers and technically ruled separate countries, Castile and Aragon respectively. Their daughter and heir Joanna, while the official Queen, did little ruling. Her father Ferdinand and her husband Phillip Von Habsburg ruled in her place for she was deemed mentally unfit to rule. Queen Mary was supposed to be Co-Rulers with Phillip II of Spain, he gained some governance power over England though Mary held the most authority but Mary had no influence Spain. Queen Elizabeth seems to have been answered before. William and Mary were also Co-Rulers with William additionally being the Stadtholder of the Netherlands. Queen Anne married a Prince of Denmark but she held power. Queen Kristina of Sweden never married and Anna of Russia's husband died. Elizabeth of Russia married a Cossack. Catherine the Great deposed her husband and engaged in rather interesting affairs after that. Isabella II of Spain who married the Duke of Cadiz held power of Spain as the Duke was more of consort. Queen Victoria was also the sole ruler of the United Kingdom and Albert her consort though she remained un married after his death. I also forgot Maria Theresa. Given the Holy Roman Empire and Austria's tradition of Salic Law, her succession was rather rough the War of Austrian Succession occurring. She was supposed to share power with her husband the official Holy Roman Emperor and her son though she was the de facto sole ruler. It is said when her husband disagreed with her during state meetings he was dismissed and she made the decisions.

Mary I of England is an interesting case. She inherited the monarchy prior to her marriage to Philip of Spain. At the time, English common law jure uxoris declared that all of a woman's property and titles would pass to her husband upon marriage, so there was concern that if Mary were to marry a foreigner, control of England would pass to a foreign power.

Queen Mary's Marriage Act was drawn up to dictate how power would be officially shared. Philip would be granted the title of King of England, but only while Mary lived. He would not retain the title if she died first. Documents had to be signed by both Mary and Philip and he essentially wasn't allowed to make official, independent decisions that affected the country. At the time of the betrothal, Mary outranked Philip, so his father granted him the kingdoms of Jerusalem and Naples so his rank would approach equity with Mary.

The latter was pretty common. High ranking nobility very rarely married without the consent and direction of the monarchy and titles were often granted or shuffled around to make more promising matches.