The only one who could answer that question would be Naoko herself. I have no idea myself. But I have seen the fan's coments! My Favorite Fan Reasons That The Starlights Change Sex:
10. So they would never be without a date
9. To annoy the heck out of us
8. To give DiC another dubbing hurdle
7. Naoko was drunk off her something-that-rymes-with-crass and when she sobered up it was already published
6. Because the idea of MEN in tight leather bikini's and bitch boots just didn't make the audiance jump for joy
5. They had to have SOME kind of power (I say boo to this one! I LOVE YATEN!)
4. NO ONE wants to see three more Tuxedo Kamens!
3. Shock Value
2. The result of Naoko's past experiments with Pez and Tequilla (I think that could account for Chibi Usa also)
And the Number ONE Reason...
Madonna is thier Agent (Now who would have guessed that?)
*several of these were taken directly from "The Sailor Moon Top Ten Lists." I lost the URL but if I ever find the adress I will put a link to it here.