There is a misconception among Western as well as Asian Buddhists that the Buddha taught that a male rebirth is more suitable for achieving Enlightenment, than is a female rebirth. But there are sutras that teach that male and female are irrelevant, as they are just as much a construct of the mind as any other phenomenon. One of these is the Vimalakirti Sutra.
Chapter 7 of the Vimalakirti Sutra, a long treatise on Sunyata (Emptiness), concludes a teaching in this vein with the words, "In all things, there is neither male nor female".
The Buddha's disciple, Sariputra, asks a goddess why she doesn't change her form to male. She responds by magically transforming him to a female, and herself to a male, and asks him why he doesn't change himself back to male. Sariputra responds that there is nothing to change. With this experience, he learns that male and female don't exist on a supramundane level, and therefore a female rebirth can't be an obstacle to gaining Enlightenment.