The Second Mango by Shira Glassman

Queen Shulamit never expected to inherit the throne of the tropical land of Perach so young. At twenty, grief-stricken and fatherless, she's also coping with being the only lesbian she knows after her sweetheart ran off for an unknown reason. Not to mention, she's the victim of severe digestive problems that everybody thinks she's faking. When she meets Rivka, an athletic and assertive warrior from the north who wears a mask and pretends to be a man, she finds the source of strength she needs so desperately.

Unfortunately for her, Rivka is straight, but that's okay -- Shulamit needs a surrogate big sister just as much as she needs a girlfriend. Especially if the warrior's willing to take her around the kingdom on the back of her dragon in search of other women who might be open to same-sex romance. The real world outside the palace is full of adventure, however, and the search for a royal girlfriend quickly turns into a rescue mission when they discover a temple full of women turned to stone by an evil sorcerer.