So far as royal mistresses who marry a king is a topic of discussion, check out the career of the beauteous and influential, and much despised Elizabeth Woodville 1437 - 1492. She was the lover, then Consort, of Edward IV.
Just goes to show how much more power a king back then had... Or how much more liberal the morals were then. A lot of what we consider today of the very strict codex of the past was only introduced by queen Victoria - lots of her ancestors surely led a more than morally lax life.
As for marrying equally: I doubt this was so important a point in England and Scotland in general in the past. It was more a thing of political neccessity for the English and Scottish kings to form alliances - the English tried to preserve their French inheritance from Eleonor of Aquitaine and their influence in the French and Flemish provinces bordering the North Sea and the Channel while the Scottish kings searched France's back-up against England. So marrying into the European Royal families was important - as it was later to marry into the German Houses of the North to protect Hannover from Prussia.
Only after WWI Royal alliances were not longer important as political means and immediately the Royals started to marry commoners - first ladies from noble families and lately middle class girls.
BTW - Elizabeth Woodville at least was the daughter of a Luxembourg-princess, who was not only cousin to the Luxembourgian emperors of the Holy Roman Empire but whose sisters had married important dukes from the reigning dynasty in France, who owned lands close to the Channel and the North Sea (Bretagne and Maine). Elizabeth's uncle from her mother's side was the enormously influential Connetable de St. Pol, who through his marriage to Joan of Bar controlled the Channel-harbour of Dunkerque. The Connetable's son from his marriage to a Savoy-princess became Great-Chancellor of France during the time Elizabeth Woodville was queen of England - quite an important relative!
Elizabeth thus brought enormously important connections to the French and Germany dynasties to her husband and her son-in-law, the first Tudor-king.
In a way this marriage was as dynastic as the others - Jaqcueline of Luxembourg, her mother, had married into the English Royal family first, so was a dynastic bride herself. Her first husband was the third son of Henry IV., John of Lancaster, duke of Bedford, the infamous duek of Bedford who had Jeanne d'Arc tried and executed. When he died, Jacqueline married Elizabeth's father Richard Woodville.