What do you think the perspective of the author is (in regards to his book The Persian Boy)?
Of course, Renault's story concerns Alexander the Great and his acquisition of Persia (hence the title), and she believes it very important to state the truth even within the confines of historical fiction. She focuses on both Greek and Persian customs and focuses mostly on the concept of love as one of the universal human feelings. Further, she believes the Greek concept that the agape love between males (as well as agape love between females) to be the higher form of love as opposed to the sexual eros (and even the filial brotherly love). Therefore, a certain sort of "bisexuality" was the ideal.
Alexander, of whom men tell many legends, lived by his own. Achilles must have Patroklos. He might love his Briseis; but Patroklos was the friend till death. At their tombs in Troy, Alexander and Hephaistion had sacrificed together. Wound Patroklos, and Achilles will have your blood.
Further, through her work, Renault talks extensively about the reasoning behind the Greeks loyalty to Alexander and the fact that he most likely wanted to unite the Greeks and Persians (and not actually take over Persia). The reason for this goes back to Alexander's (and Renault's) idyllic love: Alexander had a great love for one of his former, male, Persian slaves named Bagoas.
Renault wishes to make clear Alexander's desire for the united Greek/Persia (in the form of the "Greek-Persian Dynasty") by showing that Alexander did marry Darius' daughter.