Why are all members of the Kuja tribe born as women? On chapter 515 they mentioned all Kuja Tribe women always give birth to females.

How is this possible? Is there any explanation about this?
No explanation is ever given beyond the word "mysteriously" but it isn't as ridiculous as it sounds. 

In the real world, environmental factors such as the temperature and diet of the mother during pregnancy can affect the likelihood of a boy or a girl being born. Of course it rarely gets above 55-45 one way or the other, but in the unique environment of Amazon Lily it is possible that the odds of having a baby girl is completely overwhelming, to the point where no boy has ever been born on the island.