So is the dominance theme, but it's not being dominated that appeals to the reader of medieval romances. What appeals is the victory over dominance, which will mean equality for both the lovers. The heroine, her will as her only weapon, seeks self-value and independence against the imposing odds of a patriarchal church and society, with its powerful existing customs. Additionally, medieval romances naturally incorporate archetypes. For example, a horse and rider symbolize control over will, emotions, and sexuality, depending on how skillfully the rider handles the horse.
Knights are masterful, yet gentle, with their mighty war horses.
Furthermore, war and battle present strong images of the conflict between mind and emotion. Most importantly, the damsel in distress needs to accept the power and strength of her inner male aspect, while the knight needs to rescue and protect his abused inner feminine side. And possibly, medieval romances idealize the idea of accepting a subordinate position, by choice, for the sake of and with protection from an adoring lover, whose main purpose in life is to fulfill his beloved's wishes. A woman was property. Between them is the largest disparity of power.