You know, there is a VERY good reason why women often do the voices for young male bishonen type characters. In fact, this reason is probably why women do voices of bishonen to begin with.
It all started a long time ago in Japan...
The year was 1914, and a new theater production group, known as Takarazuka, gave their first ever performance. In Japan, traditional Japanese plays were all male, and only within the last century or so did females begin participating in theater productions (although I believe they still teach all-male Kabuki and such just to keep tradition alive). The people in the Takarazuka, however, were ALL FEMALE!
The Takarazuka Revue Company, as the name of the company officially became in 1940, was the first all-female theater troop in Japan, and remains PURELY all female to this day! The Takarazuka music school, founded in 1913 (a year before their first production), continues to train and educate young women to be actresses in the Takarazuka theater troop, and they have a PERMANENT stage set up in Osaka called the Takarazuka Grand Theater.
Now early on, mothers would often take their daughters to see the all female theater troop perform plays of their own making and ones made by others. The Takarazuka brought "West Side Story", "Gone With The Wind", and other western plays to Japan for the first time ever. In 1974, they did one of their most famous productions on "The Rose of Versailles", which is considered one of the greatest Shoujo manga ever, and a clear influence for Revolutionary Girl Utena.
Like I had said, mothers often took their daughters to the Takarazuka troop for entertainment. The Takarazuka presented women in very strong and positive light, and more importantly, made men PRETTY! The Takarazuka, as far as I can tell, was where the first occurrence of the modernly accepted "Bishonen" style appeared. The men in Takarazuka productions, as portrayed by women, were handsome, elegant, charming, and of course, often hid villanious streaks if they were actually bad people.
HOWEVER, what came across to the little girls who went to Takarazuka plays was that the men portrayed in them were the IDEAL men! Girls grew up half expecting men to be as charming, as elegant, and PRETTY as the women represented them. It's even considered acceptable that young girls formed CRUSHES on the women that played the men in the Takarazuka productions. One might say this led to some form of lesbianism, in one form or another, by encouraging girls having crushes on the pretty women playing men. An example of how that would work can be seen between Sailor Uranus and Sailor Neptune, not to mention in Utena.
Of course, the norm for that kind of thing was that girls would grow out of their crushes for the females playing men, and find themselves a nice guy who shared traits that the Takarazuka women portrayed. Find their own "Prince Charming" or "Yuki Sohma" or "Tamahome" if you will.
Basically, the bishonen and pretty boys of manga can all practically be traced to the women who played men in the Takarazuka theater productions. As for the women who are in Takarazuka, they can either continue to be in plays, or go on to other forms of acting, such as being in movies, television, becoming pop idols or other musically attributed occupations. A recent article I found showed a woman from the Takarazuka theater going on to do puppeteering for the Japanese version of Sesame Street. (Sesame Street in Japanese... THAT would be weird to see!)
For young aspiring actresses and young aspiring musicians, singers, and the like, Takarazuka is the place to be for the proper training and practice to get out into the world of theater. For the women that go to see Takarazuka, they leave impressed and awed, and it often shows in their manga where they draw the pretty boys and the like.
:record scratch: Alright, alright... I've gotten away from the main topic about how this reflects in women doing male voices in anime, even if they aren't kids. Like I said, women played men in the Takarazuka theater, so it's no surprise that they can play men in anime too as seiyuu. Their voices add that "feminine" charm that makes girls go gaga over the guys. The voice of Sailor Uranus, I recall, had a number of roles where she had to play the feminine guys.
So what about other not-so feminine characters like Goku and Kenshin (both voiced by women in the original Japanese)? For Goku, he was a kid to begin with and a goofy one at that. The woman who did his voice showed his kid like side, and was able to sound like a young kid thanks to her voice training. As Goku grew up, she compensated for his age by deepening her voice a little, but by still being a "she", she was able to help keep Goku's childishness intact with everything he says. Plus, she has literally perfected the best way to say:
"KAAAAA-MEEEEE-HAAAAA-MEEEE-HAAAAAAAAAAAAAAAAAAAAAAAA"
So in short, I believe that most women who play men in anime are cast that way for two main reasons. They are there to show the character's prettiness by having the voice match the looks, or they are there to show a character's childishness (whether they actually be a child or not).
Now all that's left is the men. Where do they fit in with all this? Well, frankly the male voices in anime often come off as very low and deep. Almost rough as though they can pop out of the TV screen and kick your ass. The most manliest voices I've ever heard in an anime were from Jojo's Bizarre Adventure. The seiyuu for that were all guys (for the male characters of course) and they exemplified the manliest of manliest roles, even the pretty boy Vanilla Ice (whose name was obviously changed in the western release for obvious reasons) had a deep and tough voice despite his appearance. After all, he had one of the most powerful attacks out of all the characters!
In Jojo's Bizarre Adventure, there is not a SINGLE pretty boy in sight, and despite that fact... the members of CLAMP fondly recall Jotarou Joestar, the most popular of the Jojo clan, as the coolest and hottest guy ever! (In the music vid, Clamp In Wonderland, the last character they show is a chibi, and then non-chibi version, of their rendition of Jotarou Joestar)
So finally, to wrap this all up in a nice tidy package -
If you want a manly man for your anime, cast a guy to do the voice
If you want a pretty boy for your anime, cast a girl to do the voice
If you want a woman, tomboy/girly-girl/etc., cast a girl to do the voice.
After all, can you think of one instance where a guy played a girl in an anime?
~Otaku-Man
I better print this out and take my time in the restroom...always a good read though 
