The reason all senshi are female- and why male starseed carriers (Mamoru) are weaker fighters- is simple enough. (Meaning: bear with me.)
A senshi holds the responsibility for her planet and dies when separated from her starseed. A senshi capable of using two starseeds would not only be responsible for two planets, but would have access to twice the power. In this way, Chibiusa is an anomaly because she can tap into both the power of the Earth and of the Moon.
Originally, when there were certain ‘rules’ being set for senshi, it was decided they should all be one one gender so there would be no breeding between them with the end result of, say, a senshi with the power of twenty planets. (Hard name to get one’s mouth around as well. [Which could, when extended, explain Sailor ‘Cosmos’ as a possible amalgamation of many planets?] Anyway…)
The reason females were decided upon was because senshi were descendants of the royal family of their planet, usually in a patriarchal system. Crown princes were too important and busy being future kings, although occasionally (in the instance of Endymion) a starseed would be passed to the wrong person. This left some less-important princess or another to be trained as a warrior to serve and protect her planet.
In the case of one such as Mamoru, the powers of his starseed manifested in a lesser, weaker way so as to be unassuming and not attract much attention; again, to avoid him being prey to some senshi who decided she wanted a daughter twice as strong as herself. Which brings to point another reason the people of the Moon couldn’t marry those from Earth- everyone knew Earth was missing a senshi so until she was reborn as a girl, the Earth would have been ‘quarantined’ to prevent senshi cross-breeding (and related mutations; albinoism, slow growth, or dwarfism for example).
So Chibiusa is basically what the powers-that-were didn’t want to happen, but oh-well-for-them-anyway.
…And we all know she dyes her hair.
