The Stories of the Lotus Sutra, p161While today we can regret the fact that early Buddhists failed to challenge the assumption that a buddha must always have a male body, it is not surprising that this was simply assumed in this story of the dragon princess.
It is an incorrect representation of the story, however, to claim that the Sutra “insists” on such a transformation. What is insisted on is the claim that “even” a girl can become a buddha. Since by definition buddhas are male, the story simply says in one brief phrase that her body was transformed into that of a male during the process of her becoming a buddha. There is no insistence. It is simply assumed to be a necessary step in becoming a buddha.